Sure there are choices. Professors of all stripes must publish. But only a very small fraction get assigned as texts. In public schools it is generally what CA or TX approved for their kids. It is a racket. They are almost all progressive and left of center.
College is slightly different because a prof can assign most whatever he wants. Given the vast majority of liberal arts profs are left of center you will get that selection in a text. If the prof wrote one, chances are it is from a left wing world view and he will assign his own and defend it. You are certainly right about students using other sources. But if the prof professes and promotes a certain view point, especially by way of denigrating other view points, you ain't going to get many students to search out other sources to support a point of view contrary to the prof when he just told everyone anyone holding that view was a moran, immoral, intolerant and more. They just take the easy way out. Use the prof's resources, mimic his world view, get a good grade and get out. Better than working hard independently, get hammered on the grade and possibly publicly humiliated.
You went to college years ago. Your experiences may have been different. Mine certainly were. I have just had three kids complete a total of 15 years at university undergrad and graduate school. It is different out there Flash. Sadly so.