They all do this now, to one degree or another. [1] [2]
The economics are even worse (because they don't get a cut from the restaurant), so these types of orders are either very high fee for the consumer, a loss leader, or both.
I've heard that on Postmates you can use the write-in functionality to order from a business that isn't on their platform at all (you order from a nearby business and in the write-in field tell the Postmate to go across the street, etc). I'm not sure if many of the other services provide a way to do that.