Suppose I pay a marketing firm money to run an advertising campaign to promote my business. They wouldn't be violating any laws by using my trademarks as part of doing this job, according to the contract we signed.

That is what Grubhub claims they are doing. They aren't doing this to random third-parties - just restaurants that signed up with Grubhub's marketing service and agreed to allow them to do these things in their terms of service. Unfortunately, in this world of EULAs that are far too numerous to ever read and understand, most of the restaurants didn't realize what they were signing up for.

This behavior absolutely scummy, but the question of whether it is legal is more of a contract issue than a trademark one.

Lawyers are expensive.