Yeah, I looked in to this a week or so ago when it started making the rounds. If you read the terms and follow how the links work, the BLM site goes to an Act Blue payment page, which then takes a portion for processing (can't recall how much) then sends the money to BLM. They probably do this with a wide range of lefty sites.

BLM probably regrets this now that the dough is rolling in, but at the time they set it up it was probably easier than maintaining their own payment portal.

But it looks like they are up front about it, I don't see what would be illegal. The fact that Act Blue sends money to candidates has no connection.