This sort of thing seems more like a self-reinforcing loop than something to be assumed innately. They write articles with a clear political slant that comes through strongly on some subjects. Potential readers who don't like that slant go elsewhere, and those who love the slant stick around, read everything, subscribe, and comment a lot.

Perhaps more concerning are those in the middle, who would say they don't have a self-proclaimed position, but aren't sophisticated enough to recognize the slant. For those types reading media like that, they tend to absorb that slant as a law of nature instead of a political opinion.

I remember once upon a time, it was considered to be a good thing for journalists to present both side of an issue. Maybe not every single issue, but at least anything important and contested enough to be worth holding a vote on. We were supposed to trust them, because they were supposed to know enough to do this. It was supposed to be dangerous and encourage extremism to present such issues as if there was only one legitimate side. I guess those days are long gone.