Who are you to decide that though?
Many cultures influence cosplayers, are you saying its only called cosplay if it came from Japan?
So then the rest of the people cosplaying from other things don't belong?
If there's one thing I've learned in my entire life its acceptance, to be able to accept otherpeople around you, their interests, their drives, so why shouldn't you? It does not personally offend you, or it shouldn't, JUST because it is from a culture other than Japan.
I saw a Storm Trooper at K-con last year, his costume was fantastic and he spent a lot of work doing it, you gunna tell me he shouldn't belong there?