The way I see it, we use the term "tip" in punting very loosely and not in the general sense of the word.

I don't even consider the money that I give to MLs as a real "tip". It's just payment for her services, that's it. If I don't pay that money, I don't get the service at all. That's not the definition of a "tip", is it?

On the other hand, when you tip a waiter, that's a real tip because it's just money you pay "for nothing more". Whether or not you paid the tip, that waiter would've served you regardless.

So there's really no point in comparing the tip that we give to MLs, to any other instances of tipping. It's just not the same thing.