These services will go out of their way to place orders against the restaurants will if they choose not to accept them. Even going as far as having employees call in phone orders while posing as regular customers.
If the problem is that the delivery services are charging high fees to restaurants, then terminating the relationship with the delivery service solves that problem. If the delivery service wants to make orders like a normal customer without any discounts, that's good for the restaurants.
Whatever their reasons for not wanting the business the OP's point was 'they can always opt out' which just isn't true.