I think degrading is in the eye of the beholder.
A fetish is defined Psychologically as "any object or nongenital part of the body that causes a habitual erotic response or fixation."
Key word here is erotic response.
Everyone has different sexual appetites. Thing is, when the majority is satisfied with their wham-bam-thank-you-ma'am gonzo flick of straight up fucking, others find it rather boring and old. But put a foot getting creamed on, and BAM! They shoot their load with ease. Fetishes are there to trigger orgasms, or feed some sexual appetite.
What most of these actresses try to do (if they themselves aren't into bondage/rape scenes/foot on face/etc) is to fulfill someone else's fantasies. Anybody also find it weird how most of the girls who eventually did porn have a somewhat similar background? Religious parents/Strict Christians?
Believe it or not, all their actions are triggered off by some event in their lifetime. Maybe, they got into porn because the IDEA that they can bang a complete and total stranger without plans of marrying him and possibly never seeing him again turns them on. Once that got accomplished, they look to tackle their next biggest fantasy.
By the way, I believe someone posted this earlier, but, I even heard it myself. Women (yes, plural) have a strange fascination with getting role-playing raped (obviously I was boyfriend with them all). I've role-played so many things that just different women were into. But I made that environment comfortable for them. But once a woman opens up, you'll be surprised some of the things they fantasize about.