Why mathematicians are boycotting their biggest conference

More than 1,500 mathematicians are demanding that their field’s most prestigious meeting be moved from the U.S.

Scientific American

Nobody will care if the conference isn’t held in Philly. Holding it elsewhere will probably make it a little easier and possibly a little cheaper for people to attend. I doubt mathematicians are part of the 1%, so cash and travel hassle should matter. And given today’s Internet, there’s going to be remote attendance which can happen most anywhere.

While it’s still convenient to gather together to discuss a field, it’s not crucial as it was in past times. Easier to do what’s best for the largest number of people.

Huh? This is primarily because travelling to the US is not worth the risk right now.
It's just grandstanding.They are mathematicians not political activists. If they want their organization to slide into irrelevance, getting involved in left wing (or right wing, but with academia it's usually left wing) politics is a great way to do that.
Anyone can be a "political activist". An activist is just an ordinary person who has had enough. Unless you believe the only valid way to influence political discourse is with money.
Sure, anyone can be an activist but it is clear that academia has been turned into an activist training centre. It is also remarkable how these supposedly intelligent people go astray when it comes to the causes they support, from supporting Hamas to defending those who'd throw them off high buildings or putting them against the wall if they got their chance.
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The Conversation

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.

"The petition follows months of trepidation about the congress within the math community. “You do not get 1,500 signatures in 10 days without having many, many mathematicians already registering their complaints to their professional societies and to the ICM organizers,” says Ila Varma, a mathematician at the University of Toronto and one of the petition’s co-authors."

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ICM's peak attendance is around four thousand, so 1,500 would-be attendees signing a petition to move the conference in ten days is pretty authoritative.