Perplexing

The more and more I live in this lifestyle the more and more I'm perplexed by an issue that is no bigger or smaller than one makes it really.
If a Top/Dominant/etc is competent in a certain set of skills, they are basically expected to perform said act for whomever comes a-calling to get said skill, aka "Service Topping." It is a great thing to do for the sake of helping out new people figure out what they want but there is much more to that as well.

As a "Service Top" you're expected to not only execute said act without any issue or straying from the "right way" but also expected to offer aftercare to ensure the bottom's well-being afterwards, even when they've said they don't want it, and basically be perfect during the entirety of your interaction with each other.
It is rare that "Service Tops" get anything in return other than "hey that was great" if they did well, or a barrage of bashing should one tiny thing go off schedule.
If a "Service Top" changes their title to "Professional Dominant" or "Pro Dom" for the purpose of getting a little something in return for providing the same service that they would while service topping, they are suddenly a prostitute, or any other number of horrible things, simply because they want to make play more worth while for them.
Hence the perplexing issue: Why is "Service Topping" completely acceptable and expected but being a "Professional Dominant" who does the exact same acts, suddenly taboo and horrible?