How about because you believe in it?
From Gunner:
Progressives cannot allow libertarians to lead the fight against the misuse of eminent domain
From Chris Kromm:
It’s time for all progressives to make Gulf Reconstruction a top priority.
So progressives should only take a stance when they’re trying to teach conservatives a lesson? Try doing stuff because, you know, you actually believe what you’re doing is the right thing to do.