They haven't been able to(as yet) block out talk radio or the Fox News channel, which have the added benefit of not having to be able to read.
Good point but all politics are local and these shows have a national focus.
As an example here in Fresno, Harry Papas owns the local Fox broadcast outlet. Papas is a died in the wool liberal. He refuses to allow Fox's news report, tailored for nightly broadcast television, to be published in the local market. He even relegates the Sunday morning political round table, with Tony Snow, to the 6:30 a.m. broadcast slot if there's the least excuse of a programing conflict including infomercials.
In substantiation of your point the two most popular, local, talk radio programs in the Fresno market are conservative in nature. Strangely, the largest is a CBS affiliate. This presents an interesting arrangement. The hourly national news, which drips of liberal spin, is then critiqued and criticized by the talk show that follows. To date CBS has apparently allowed this insubordination because the game sells.