Unions are a pretty consistent supporter of the Democratic party, yes so is capital but the DNC can't afford to just tell unions to f off. Increased unionization will lead to more D votes, for good or ill.

Edit: said another way, unions are to the DNC what evangelicals are to the GOP. Yes their interests are different from capital but the parties still need their votes.

Multiple unions have threatened to end contributions to Democrats that don't support the PRO act. Some are listening.