I suspect that while the non-US teams are at YC, some of them are technically 'tourists' (i.e just on holiday). They don't need a visa to enter the country so it's a case of hear-no-evil, see-no-evil.
If they decide to stay in the US after YC, then they have to deal with the nightmare of immigration and get themselves proper work permits.
For those who need a tourist visa to even enter the US, I have no idea.