Pretty much what the guy above me said, as simple a response as it is. I've seen escorts asking for a massage and a lot of the time you will just get either more of a rub or a mild massage that will turn into sex all too quickly.
Personally I haven't been to a thai massage that doesn't offer HR. They will never openly advertise it, some will just ask if you want extra, hint at it or if they ask whether you've been before I've usually had that as them basically asking if you know that they offer other stuff.