If you're talking about the guy that tanks up on Atlantic Bucks and then is overweight for takeoff or landing, or who has to fly at a ridiculously low altitude to burn enough fuel to make landing weight, then yes you are correct in using the term unethical. But that term should be applied to the unprofessional, weak-minded pilot who allowed their fueling decision to be influenced by an outside factor, not the program itself.
If my taking an extra 10 gallons results in my coworker getting an extra buck, you can bet I'm going to take that extra 70 lbs, unless it pushes me over a limit or the fuel is more expensive than at our next destination. I don't operate so close to the limits, when given a choice, and I usually prefer to have a little extra fuel on board anyway. I don't know of any accidents that resulted from over fueling, but I do know of situations that were made worse by a lack of fuel and therefore a lack of time to fully assess the situation. I benefit from flying an airplane with phenomenal landing performance, so a few extra gallons does nothing but lower my stress levels. Taking an extra 500 gallons would certainly be excessive and borderline unethical if there wasn't any other reason than Atlantic bucks. It seems your line between a professional pilot and an unethical pilot is a pretty fine one, and doesn't leave much room for contingencies/delays that your flight plan didn't account for.
Just make your decision, stick to it, and if the FBO wants to give you a few bucks, a couple of steaks or whatever, AFTER the fact, then just be thankful and enjoy. It's pretty easy to make those decisions without outside influence if you try.