It would not surprise me if "we can use your brand for advertising" is explicitly mentioned in the GrubHub/DoorDash contracts. I think the overall problem is that restaurants don't have much leverage; their main option is to refuse to cooperate.

Now the (gradually becoming less common) practice of auto-creating order pages for restaurants that didn't agree to be a partner, I would agree should be totally illegal, or at least needs a very large disclaimer that the restaurant is not involved.