I've been to Mexico actually and there was a street vendor on pretty much every corner selling sombreros to gullible tourists. The last time i went to Oktoberfest, some Germans felt (jokingly) insulted if you didn't wear a lederhosen.
Hell, go to pretty much any country and there will be street vendors and souvenir shops trying to sell you stuff (cheap knock offs mostly) that has some cultural significance to the indigenous people.
Not trying to insult anyone here, but i just have a hard time grasping why people find this so upsetting. Now i am from Norway, if someone from abroad decided to dress up as a viking or dress up in a "bunad" (traditional folk costume) i could not imagine being insulted by that. I remember last 17th May (our National day) a Muslim girl had sown her own bunad/hijab crossover. She did get some abuse, but that was from racist thugs and right wing nutters, as most people (me included) found it great that she embraced Norwegian culture whilst showing her own.
Now obviously there is a stuff like dressing up in black face, an SS solider or a member of the Klan that is offensive. But dressing up as a mariachi, a samurai, a viking or a cowboy i don't really see the issue with. I think that if a culture is strong enough to have developed such identifiable traits, then it's strong enough to survive some kids running around with sombreros and maracas and having a good time.
I just find this mindset of: "This is mine, your a foreigner, you can't have it" confusing coming from the left side, as it' usually something you would expect from the polar opposite. Just because you use something from a different culture does not mean you seek to belittle or mock said culture. I don't know man. In this day and age with the resurgence of the far right and an escalation of conflicts between ethnic/social groups i think this mindset does more harm than good.