Yes, because Polymarket offers sports markets, which are still illegal in the US (for US users, anyway). Kalshi has been careful to offer markets in basically everything
except sports.
It should be obvious what happened here: Polymarket has been offering grey or black markets to users, using only IP address as the discriminator, which is very easy to work around. They've operated under the Feds' radar for quite some time, and the Feds noticed them after they started getting significant press regarding the election.
The election is the reason they got noticed, but the likely crime itself is sports, not elections.