If a restaurant already has a contract with GrubHub and somewhere buried in the contract it gives GrubHub permission to do this... then it's still shady, but owners should also be reading their contracts closely -- and for owners who don't have a website at all, it could even be beneficial.
BUT... if GrubHub has done this (which they reportedly have, although they seem to be denying it) for businesses they haven't signed a contract with... then is that not highly illegal? There have to be laws against false representation in business, no? Whether cut-and-dried trademark infringement, or otherwise?
If that's the case, I hope they get a serious class-action suit against them with massive punitive damages. It's deeply unethical and anticompetitive, and basically smells of extortion.