It takes 3 seconds to tell them you won't attend once you're already on the phone . It realistically takes a much longer time to phone tag and then determine $ allocations.
How's it self-serving to prolong your own final determination (whether admit or reject)? That's done at the expense of everyone involved including yourself.
Your argument specifies nothing about the thing at contention here, IE: Asking for X if accepted. It was said that if you wouldn't 100% attend, you shouldn't stay on the waitlist because they will very likely not meet your obligations. Well, if that is the case, then it will literally take 3 seconds once on the phone for them to render a decision. In fact, you concede that they may match the obligation by stating the added time period for money allocations to take place. So maybe that's why it's self-serving?
Your argument more specifically attacks why anyone would stay on a waitlist once accepted to another school. You stay on the waitlist because you might get accepted and attend in the right circumstances.
This assumes that the positive value of the very, very small chance you might get sufficient aid in July outweighs the negative stress or any bureaucratic difficulties from staying on. From a purely utilitarian point of view, yes, you should never drop off the waitlist, as there's no tangible advantage to doing so. But many people drop off waitlists when they want to "just move on with their lives", so to speak.