I think one of the reasons you have struggled with this concept, is I think its a mixing of words.
I don't think people use PSF as an "actual source of news", rather when they say that, what they mean is they are informed of news-worthy events by the PSF. This means, when something newsworthy happens, it shows up in a thread here - generally with links to the actual "news" and then discussions around that event.
So, as an example, the thread about the assault in the girls bathroom - that generated a thread, with a link to the story itself. For many people here - that is where they first learned of the story. And then, both sides have argued their respective points of views - giving everyone who reads the thread a pretty good indication where the battle lines are drawn, and they can decide where they fall.
So, the PSF is not an original source of information - such that it could be weighed left, right or center. People who post in here are pretty blatantly left, right and center, and the opinions they provide are colored that way. If you have been here long enough, its pretty easy to identify where people are on any given issue, and while some are die hard in either direction, most here fall towards the middle, and the specific issue guides how far left or right of center they fall.