Because really the networks pay for a season, but they are not part of writing. They may say you can or can’t show this, but overall the writing and ideas come from the staff on the show. Lets face it, people these days want to get their views out there, so many writers put their views in their work. It happens. I have been watching Law and Order:SVU re-runs and wow, they tend to touch on a lot of controversial subjects, I was surprised.
If the owners of the networks fully controlled everything played, I doubt a conservative based owned network like fox would play simpsons or family guy – lol