We're talking about assuming that a site's default user is from the US. I'm saying if 49% are not, then that assumption seems ethnocentric. That doesn't require every other user to be part of a monolith of non-Americans, they all share the trait of being... you know, not American.
That's a big chunk of your users that don't conform to your default use case. If this was about anything else you would not a default at all in that scenario, but that's not what happens. Also, it's not blamed on the "average American", it's being blamed on Americans as a whole, culturally, on the aggregate, which is fundamentally different.