In September of 1787, the Constitutional Convention was debating the possibility of adding a bill of rights. It wasn't going well. The list of those things that the proposed Federal Congress would be forbidden to do was whittled away to practically nothing. Why bother when most of the delegates agreed that no one would DARE pass laws to abridge the likes of Freedom of Speech, Assembly and the Press? However, a few things were held to be so egregious and dangerous that they were retained in the original constitution.
Among these were: No Bill of Attainder or ex post facto Law shall be passed.
Now what kind of laws might these be? Okay, let's take an example: Let's say that a corporation nobody likes, AIG, has given a bunch of people nobody likes, their executives in charge of toxic assets, bonuses that seem to be waaaaaaaaay too much in light that the company was driven into the ground and is in government receivership.
Now let's say the checks were legal and that they were already paid.
Got it? Good.
Now let's say that Congress, in it's righteous anger, decides to get the money back. How do they do it? Well they pass a law taking it from them. Just that. "We don't like you or what you did, so you're going to be punished."
It was legal at the time. Thus any "tax" on income that wasn't in effect at the time the checks were paid out is ex post facto after the fact. The constitution is very clear on this: If it's legal now, then it's legal now. You cannot say if Congress later bans it then it's not legal now. Now is now and the law now is the law now. It sounds redundant, but the AIG "tax" bill passed by the House (and supported by President Obama) is retroactive. It taxes income at a special rate retroactively. Taking property without due process of law.
Another thing the bill does is criminalizing a specific class of people. The aforesaid executives, and punishes them because of who or what they are. They are attained guilty by Congress and are being punished for it. Not because they broke a law but because the law broke them.
Now you may not like these people, and may think that the law is just and AIG is run by a bunch of scumbags. However, the reason why the Conventioneers put this in the constitution was that they didn't trust future legislatures.
Congress can't ban cell phones retroactively and then throw everyone in jail for having one under the constitution. They cannot pass a law saying Guys from Alexandria Virginia are all evil and should pay a million dollar fine each and spend a year in the clink.
Yet, this is exactly what the House of Representatives just did. This is what the president, who took the oath of office just two months ago, endorsed.
If we can do that, then Congress might as well throw Rush Limbaugh and Sean Hannity in jail for being jerks and get it over with. Then confiscate Rupert Murdoch's business holdings and take away his television. Then they could take away YOUR television and give it to the poor!
Some things you have to just let go. The AIG bonuses were scuzzy as hell, but we have to live with them. Freedom is too precious to throw away even for this.