I've always wondered about this too and found it strange to get my head around. I mean, like the OP says, a girl will suck your dick or let you put it inside her pussy, but not let you kiss her. To me kissing is just not that big of a deal when compared to actual penetrative sexual acts. It doesn't mean romantic 'love' to me, it just means a bit of sensual fun. I think, yeah, as many have said, it is about the level of intimacy a prossie can take whilst doing her job, and about hygiene. But I think it's also about money, some will say they don't do it and then spring it on you as an extra in a middle of fucking them, as happened to me on a recent punt. I also mean though that it's about money in terms of how much the prossie values the money she is being paid and how much she is willing to give back in return in order to be a prossie who gets lots of repeat business. Some, I feel, just don't particularly care, they're doing it just to get the money by selling their body, they could be well meaning people, but this just isn't the profession for them, or they could just be somebody who isn't very nice at all or a giving person.
There could be a thousand different reasons. But trying to understand what is going on inside the mind of a prossie is probably a fruitless endeavour. It is up to them what they are willing to do. But if they advertise it, then well, they should do it as far as I'm concerned, unless the punter is unclean, has a bad attitude, or there is some other good reason to deny services.