At the time, Dog was a nobody. He had embarrassed Mexico officials by catching a fugitive and possibly cut off a source of their income. He is a bounty hunter. It doesn't take much imagination to think that the guards might let that information loose. Then it's just a short shrug, "We are a poor country. We had to put Chapman with everyone else..."
Have you ever dealt with Mexican Federales? Personally, I mean. I have. Their corruption is LEGENDARY.
Have you?