I should have remembered that people in these boards get offended easily.

Let's go over this once more. A guy trying to set up his band, most likely in a big hurry, looks over and notices a chair not in use at your table. Your table right next to the stage. I assume there was no place setting or glass there, am I right? He takes the chair. I don't see the problem. Not one little bit. Could he have asked? Yes he could have. Was it rude that he didn't? Absolutely not as the chair was EMPTY, therefore not really yours in the first place.
Since you've told this much, could you please tell the rest? Maybe we could then understand why you were so upset that it ruined the rest of your evening.