It is interesting. These recently coined terms such as "social justice warrior" and "white knight" seem to be used as linguistic magic wands. Powerful spells the mere invocation of which should poison the well of ones opponent. Much like some earlier terms tiredly bandied about by those of a different political bent.
I understand what you are saying that SJW has been bandied about as a negative, but I am more neutral about it, and I really don't consider the label to be as negative as some want to make it out to be.
The term White knight is way more fucking legitimate and valid regarding the behavior of someone else, and a white knight seems to be someone who is getting involved in some other person's concern when the other person may certainly be sufficiently capable of defending himself/herself. Don't get me wrong, I don't disagree with the power of some person to actually designate actual appointed representatives and/or spokepersons - however, a white knight seems to be a self-selected representative of some cause that may only be tangentially related to his/her own interests.
A SJW is a communist. We use a different term at the moment because humans are frightened animals that refuse to accept that we are sliding back towards state totalitarianism.