I'm going to guess that for many signers-- or at least the US ones-- their opposition to the United States and "its unbridled hatred" doesn't extend to not accepting funding from the US taxpayer.

Entry requirements and the overhead of dealing with visa hoops are a perennial problem for international conferences, nothing new-- and presumably a part of why it hasn't been held in the US in recent memory. But the language on this petition is particularly extreme.

Ain’t much US taxpayer money going to mathematicians and I think that if any goes overseas it would be to US citizens.