How many non-Irish people wear green and drink green beer and eat corned beef and cabbage on 17 March?
How many non-Germans celebrate Oktoberfest in the fall?
Cinquo de Mayo, anyone?
I live near enough to Chicago that I've been to some of their very Polish-themed fests.
Ethnic bits being absorbed into the larger cuture happens. A lot.
There used to be the idea of "the Great Melting Pot". It was all the rage for a long while in the U.S. I believe it was in Canada too. And if one finds that bits of their ethnic identity have made it into the larger cultural zeitgeist then, congratulations, your ethnic group is finding genuine acceptance within the larger culture.
Nothing is being taken from anyone. Everyone still has whatever ethnic culture they feel they have the right to. But to say "It's mine, and you can't share it", that kind of behaviour makes the person saying that an outsider.