I've always thought it was amusing that the Doms that don't offer sexual favors want to set the rules for everyone else. I can't count how many times I've seen a Houston Dom complain about this on various boards. I used to regularly participate in the BDSM scene. Of course, most Doms don't offer favors, but there is no rule set in stone.
There always have been and always will be Doms that offer extras. It's also a YMMV kind of thing as well. I do remember the first time a gorgeous Pro Dom let me go down on her, after the session was over we shared some small talk. She said something to the effect of, "Licking my boots does nothing for me." If I'm going to have you lick something, it ain't gonna be my boots!"
Offering extras isn't for every Pro Dom, but trust me, there will always be a great demand for those that do.