I don’t think many educated, upscale people listen to radio infomercials for cancer doctors or cosmetic surgeons or diet doctors or for stock trading classes or for dehydrated fruit and vegetable pills. So, radio stations running them risk losing listeners to other stations, to other media, to the Internet or to alternate activities.
Those shows are running on days and in dayparts where there is little use of AM talk radio and even less conventional ad revenue available.
As long as the infomercial sponsors make money, they keep buying. And that is revenue that stations that carry them would otherwise not have.