> I'm not going to go hungry or cheat on my diet purely out of politeness.
It's not politeness.
If we distill "all you can eat" down to the basics, you take a small amount, you eat it, only then do you get more.
It's generally fine to reorder those actions, such as by getting more food upfront, but the outcome needs to be the same as the basic version. That's the business deal being made. If you make a deal with the intent to break it, you should feel guilty, outside of exceptional circumstances.
If they're not willing to serve something acceptable to you, that's bad of them, but it doesn't mean you get to break the rules. Even if you "had" to be there for some external reason.
Also ejecting someone, or cutting off their supply, is probably nicer than charging them for eight pounds of extra food.
And again, this only applies to all you can eat situations. Not other restaurants, not plane meals.