The Washington Post has an inviolable rule: A reporter on assignment is not permitted to break the law. Period. No exceptions. Depending on circumstances, violating this rule can be a fireable offense.
The writer believes that if he declines the pipe, he loses his last chance at establishing the sort of trust and rapport he is hoping for -- that however reasonable his excuse was, he would have established himself as aloof and censorious. He believes that taking the pipe might well finally persuade Bob that he is someone to be trusted.