You do not understand the US primary system, in it only the Democrats are running against each other. Biden is popular and will win, the uncommitted vote is to show how many people care deeply about ending the genocide of the Palestinians. It is safe because there are no fascists as front runners in the US democrat primary.

Due to the two party system/first past the post the US primary is the only safe place to show discontent via vote on a national level. (And only during a 2nd term year)

The genocide of the Palestinians is a very, very horrendous action that is still taking place as I type. This has motivated a significant chunk of people to protest vote, in a way that does not enable fascists.