I suppose it is possible that they got their tickets from the allocation given to the players rather than the club top slicing them from the allocation that went to points holders. Even so, all of them turning up wearing Barca shirts isn't helping matters and will only lead to suspicion and anger.
As someone else said, many of us have been football tourists at some point but we wouldn't go wearing City shirts at a Barca game. I'd also say that if you're going to a game as a tourist, at least try and support the team whose fans you're with. On my mate's stag do in Prague in 2006 we went to see Slavia Prague play while we were there. We supported Slavia and joined in with some of the chants as well. 2 years later 4 of us did a few days in Munich for the Oktoberfest. On the Sunday we went to watch 1860 Munich play Ingolstadt. 3 blues and 1 red in the home end and all of us supporting 1860, not turning up in another club's shirt and showing no interest in what was going on.
There isn’t a rule saying fans of other clubs shouldn’t wear their clubs shirts at City matches. I’ve seen loads of different teams shirts worn by tourists at the Etihad. But there is an accepted rule amongst fans that fans of other clubs shouldn’t wear their team’s shirts at other grounds, especially not in the away ends. It’s a sign of disrespect.
Like you I’ve been to a few European games whilst on holiday. I would have never worn a City shirt to those matches. If you go and watch another team, you usually support the home team, or be neutral, if you don’t have an affiliation to either side.
