One lady I used to see had a theory about why some clients respond to questionable classified ads, they're thrill seekers who like the element of risk involved. I can think of ways of getting your kicks that don't involve getting robbed or worse but that's just me.
I have a slightly darker take, I think that for some of these guys, they go into these situations looking for trouble, they want a fight. Again, I can think of ways of dealing with any anger management issues you may have that don't involve ending up in a jail cell or hospital but, again, that's just me!
Risk is stimulating for a lot of people. At the low end, there are things like smoking cigarettes, shoplifting or not declaring a purchase when passing through customs.
There was a certain appeal to the sex industry when street walkers who weren't crack hoes were plying their trade on Jarvis Street between Gerrard and Carlton, on near the corner of St Catherine and St-Laurent in Montreal - letting the little head decide, yeah or nay, in ten seconds or less.
For those gay guys mentioned in the post you quoted, I think there's an element of masochism, and also a yearning for sympathy, which is sort of pathetic. They don't expect to be seriously injured by a street person who rebuffs their advances, but they do hope to be visibly injured, so friends and acquaintances will ask about their ordeal, which might be embellished in the retelling.
The junkie just wants their cash, jewellery or portable electronics, but he doesn't want police involvement; they likely already have his fingerprints, mug shot and DNA on file. What he has going for him is that he's almost certain to make bail if he didn't hurt the guy too much. Many people would be more likely to report that sort of attack to a local TV news crew, rather than to the cops.