This is not speaking "for". It is speaking "about". About is always someone else's job to perform. "For" would be writing it up yourself, and handing it someone else to read.
If an activist speaks up about a group of people being marginalized, they're speaking 'for' those people. 'About' would be if it was descriptive speaking (perhaps an anthropologist's article) but if it's in support of those people than 'for' is the right word.
BTW the object doesn't need to be people, it could also be animals or the environment or even abstract ideas like liberty or justice.