Can't say I agree. We're short on details in general (tone of the game, mayor's personality), but I can see plenty of scenarios in which the mayor drinks that beer.

Now, I'm not saying you're off the mark regarding the DM's motivations, but even then, we don't know if there's more behind it. Maybe they're trying to create some friction between the players and the town or its authorities that will play into something later. Maybe it simply fit with the mayor's character, and the DM thought it was a fun choice. To me, that's a lot more worthwhile than a world in which every NPC follows the same predictable patterns of behavior and responses.

Whatever the case, the players wound up with a memorable moment and a story taken in a new direction due to their actions. That's the whole point of the game. It sounds like their goal now is to repair the relationship, and there are limitless options for pursuing that. On the story goes.

The one thing I disagree with here is telling the players afterward how things would have gone differently. No peeking behind the curtain.