It's prostitution mate. Your concern seems to be you suspect prossies do not enjoy swallowing? Sounds pretty arbitrary to me. Why is kissing, sex, anal or anything else any different? Have you seen all of the people they do all that with...
They perform these acts on total strangers for money. And what matters in reality is just that it is agreed between consenting adults. You may hope they also enjoy it (and obviously, a good prossie can convincingly fake enjoyment if necessary), but that's not the premise on which you book or a serious concern (for any of us - if it was, we wouldn't be paying!).
Some say it's morally dubious, but this is the reality of punting. You may understandably wonder how they do this job without being disgusted, and of course many women would be, but I guess the answer is just that people do lots of things for money. It isn't a big deal to them if they're prepared to offer it to anyone.