> I meant "solely from tips", obviously.

That was not obvious. Nor is that a common practice

Let me restate what the OP said. (quotes used for paraphrasing, not for quoting)

"Restaurant owners set a price on their dishes, because if everyone payed at will, the owners know they would make less money. Therefore we can assume owners know that paying at-will does not optimize earnings. Therefore, it's insane that they make their staff work for pay that's at-will".