"Trust me" isn't exactly a convincing argument...

I agree with your second sentiment (although I wouldn't generalize to "scumbags"), but there are a lot of people out there doing jobs that are extremely unpleasant, for far less money than prostitution can make.

If you don't have to fear your customer because the legal system has your back (making you independent of pimps and the like), prostitution is a possible way to make a significant amount of money - on your own conditions, in your own time, without the fear of being fired and all the other stuff a normal workplace imposes.