American fighters are born with a silver spoon in mouth. Home advantage. Fact.

[Fenster]

So by THAT rational, why are people in London cheering for Tyson Fury when he fights there?

Because he has fans?

And Broner doesn't?

Also let's not forget that the average fan with nothing to lose or gain in a fight quite often just want to see someone regardless of who knocked out

Broner has American fans Like Tyson Fury has British fans. Not every Brit cheers for the Brit like not every American cheers for the American.

To say "Americans don't support fighters just because they're American" is rubbish. It depends on circumstances and individuals just like anywhere else in the world.