There are a couple of reasons I routinely tip sex workers regardless of the local tipping customs. One is a matter of principle/philosophy; the other is a practical matter and an issue of taste.
The former is that the fees sex workers charge their customers are basically/mostly determined by market forces, by "supply and demand", just like the costs of nearly all goods and services in a market economy. However, I think sex workers should (deserve to) be paid above market because of the several ways in which their job differs from that of almost all other workers.
In most places their work is illegal, and they bear by far the greater share of the legal consequences (compared to their customers I mean). In almost all places their work is moderately to severely socially stigmatized, to the extent that they almost must lead a "secret life", normally under a false identity, that significantly isolates them from their families, friends and lovers (present and future) and which creates a "hole" in their personal and professional histories in most cases. They run far greater risks of contracting infectious diseases of all sorts (not just STDs), of encountering criminal violence and of suffering from work related psychological disorders. The requirement of their jobs to quickly (often within minutes) become intimate with strangers also requires remarkable courage and self-confidence that few people possess. Most people feel nervous, often quite nervous, the first time they get undressed in front of another person for the first time, even if it is someone whom they know reasonably well and to whom they are attracted. Try to imagine having to do that multiple times per week or day with someone you've just met and whom you may find quite unattractive...but of course it doesn't stop at taking you clothes off.
You get the idea by now I hope. Sex work ***could*** in principle be just another job/profession. And it ***could*** in principle be regarded as noble work, like that of a health care professional or a teacher or a firefighter etc. But it ***isn't***. Societies, to varying degrees, go out of their ways to make a difficult job, one that benefits other people, even harder and more stressful.
Personally I am extremely grateful that there are women who are willing and able to do sex work, and I want them to be rewarded beyond the market's hard economic "justice"/"fairness", so I do my tiny little bit toward that goal with tips (though not only with tips). Imo, all thoughtful hobbyists of character should be doing the same.
This is long enough, and I have run out of time. I'll put my second reason in a separate post another day soon.
-Ww