I think to most of those people it's like the US is trying to "steal" the name America from the rest of the continents, but it's really only the Spanish speaking parts of the Americas that feel that way about it. But they can have it their way in their language, and it doesn't detract from anything. It's like getting pissy because some cultures count continents by separating Europe from Asia and others have Eurasia (without even getting started on Afro-Eurasia).

Are you from the USA?

I never understand why USians get really pissed about them being called USians.