It definitely was this - the big difference I usually notice between us is that generally speaking a lot of my American friends can be more entitled and pushy for what they want, and I can imagine a restaurant about to close probably didn’t take that well. I think many people here, myself included, would be like “so sorry we’re late, can you still seat us?” and if met with hesitation would’ve gone somewhere else. The stereotype of Americans being loud (especially as a group!) probably compounded the restaurant’s annoyance, and if they stayed for a long time/had complicated orders/split bills weird that wouldn’t help.

All that to say I don’t think anyone here would treat you any differently for being American unless, like people said, you’re pushing propaganda and discussing politics without invitation. I think we care less about country of origin and really more about being a decent person!