They were four million. Four million French of all stripes and colors who marched in the streets of Paris and in dozens of French cities to mourn the victims of the bloodiest terrorist attack France has known in the past century. Seventeen dead, among which the founders and cartoonists of France's most outspoken and satirical newspaper, murdered in cold blood by Islamist extremists claiming to act for Al Qaeda. 
In the midst of an exceptional show of solidarity and grief, there have been some dissonant voices to criticize the "I am Charlie" slogan, endorsed by millions of people around the world. I struggled with some of these pieces (here and here for instance). They are articulate, intelligent, sensible, and concerned with themes (Islamophobia, racism, majority vs. minority) I grapple with in my professional and personal life.
However, I disagree with them. Here are some of their arguments, and why I think they are often misled.
-"I am Not Charlie because I don't condone the cartoons of Charlie Hebdo and disagree with their message." As do millions of Muslims and non-Muslims alike in France and all over the world, who nevertheless marched under the I am Charlie banner or adopted the hashtag. No doubt many of them were outraged and disgusted by the cartoons. There is hardly one group of people, one belief, one community that has not been the target of Charlie Hebdo's incendiary, obscene, iconoclastic publications. "The whole point of Charlie's satire was to be tasteless and obscene, to respect no proprieties, to make its point by being untameable and incorrigible and therefore unpublishable anywhere else."
Have all these people whose beliefs and identities Charlie Hebdo desecrated, changed their mind and suddenly endorsed their cartoons? Of course not. Because I am Charlie does not mean you support the cartoons. It is, in its simplest expression (more fitted to today's need for short, to-the-point communication style), a show of solidarity. It is a show of human dignity in the face of terror: that one can, beyond their radical differences and disagreements with the victims, empathize with these victims.
-"I am Not Charlie because Charlie Hebdo is racist." Certainly not. Arabs or Blacks were never targets of Charlie. Quite the opposite: many people ignore the history of the magazine, maybe because English speakers are not familiar with it. In 2013, Charlie's editor-in-chief and one journalist published an op-ed in Le Monde. They reminded the readers that "antiracism and passion for equality among all human beings are and will remain the founding basis of Charlie Hebdo." The paper famously asked for the Front National, France's xenophobic far-right party to be banned, and some of its most pungent cartoons targeted FN leaders (Jean-Marie and daughter Marine).
Unfortunately, their anti-racism humor was often mistaken as racist by hasty interpretations that overlooked the details and missed the satire (such as the controversy around Christiane Taubira, the Justice Minister). 
-"I am Not Charlie because Charlie Hebdo is Islamophobic." Here again, distinctions are in order. If Islamophobia means phobia of the religion of Islam, with its sets of beliefs and sacred symbols, there is no doubt that yes. Charlie perpetuates the old French tradition of antireligious satire. (In the early days of the 20th century, Christianity took a much harder beating than Islam ever did in widely-circulated caricatures that even Charlie would have a hard time publishing today). Charlie considers all religions as obscurantist, oppressive and ultimately ridiculous. One might not like that, but free speech does include the right to debase religions and offend their followers (at least in France).
-But is Charlie anti-Muslims ? Did the paper propagate and incite hatred against Muslims as a community ? Did it conflate all Muslims and extremists who commit violence in the name of Islam ? For an answer, a closer look at the cartoons is warranted. They ridicule jihadists and fundamentalists (clearly identified as such by weapons or captions), vilify the burqa (and not the Islamic veil), excoriate a certain kind of charia law imposed by some countries - hardly symbols of the entire Muslim community, and arguably not ones with which most French Muslims identify. Granted, many Muslims feel personally assailed when their core beliefs and most revered symbols are under attack- but we all do, when our own personal creeds take a hit from our adversaries. Blasting the symbols, including the Prophet, is not the same thing as portraying all Muslim individuals (religious or not) as violent or backward. No doubt Charlie indulges in the first ; I don't see evidence for the second.
-Finally, "we should not mock or satirize Muslims because doing so perpetrates the oppression of a marginalized and endangered minority". While I am concerned by the growing Islamophobia of Western societies and the systematic discriminations Muslims are facing, I find this argument disturbing in two ways.
One, describing the Muslims as a "defenseless," powerless, weak community strikes me as highly condescending and dangerous. It is disturbing as it reminds me of the Western love for depicting Muslim women as faceless victims whose sole identity is defined by their forced submission to men. It runs the risk of victimizing the entire Muslim community instead of fostering its capacity to fight discriminations and injustice.
Two, "what is really racist is the idea that only nice white liberals want to challenge religion or demolish its pretensions or can handle satire and ridicule": all across the Muslim world, there are hundreds of journalists, cartoonists, writers, comics who put their life at risk by challenging religious orthodoxy and blasphemy laws, drawing the Prophet and making fun of what they see as outdated or patriarchal elements of their faith tradition. Ignoring them is, again, essentializing Islam as one uniform body of people with no variations in opinions and sensitivities.
They were hundreds of thousands of Muslims, Jews and Christians, among the four million who marched on Sunday. Many of them disliked Charlie Hebdo cartoons, found them hurtful and disgusting. And yet, they held banners saying "I am French, I am Muslim, I am Jewish, I am Charlie."
Like them, I am Charlie, because I mourn for the victims and for France, my country. I am Charlie, because I support free speech, the right to offend religion and the right for believers to be offended. I am Charlie, because I am against racism and discriminations against all stripes and colours of people.