It's not about ignorance. Ambiguous contracts are bad--and risky--for both parties, period. Worse, the relevant section from GrubHub's TOS appears to be written in a manner that purposely maximizes that ambiguity[0] to the company's advantage. That's an issue that people can absolutely take issue with.
Subsection (a) combines two distinct provisions in a really messy way. First, GrubHub agrees to enable its users to order food from its "ordering system/advertising service" on its website, associated app(s), and any of its affiliated web or mobile properties. Second, you have the separate provision stating that GH "may create, maintain and operate" a "microsite." These are distinct, but combining them helps give the impression that they're related.
The agreement doesn't define what a microsite. You could plausibly argue that the restaurant's page on their website qualifies as a microsite. For that matter, it doesn't define what constitutes a URL in this context, and using the term "URL" instead of "domain" seems like a willful effort at obfuscation. "Registering a domain" is a specific act with a common understanding; "obtaining a URL" can mean literally anything, including "registering a domain." They're purposely using a broader term--that they don't define for the contract--in lieu of a more specific one that's also commonly understood.
Finally, GrubHub operates the "microsite...on Restaurant's behalf." Could this create an agency relationship between a principal (the restaurant) and its agent (GrubHub)? What authority is conferred and what duties are owed? Or does the relationship something less? If the relationship is terminated, because GrubHub "[obtained] the URL...on Restaurant's behalf," will the domain name that was registered be transferred to the restaurant? Literally none of this is stated.
If you read the original article[1] about the practice, there's an added wrinkle. GrubHub gets to charge a higher commission on orders originating from their marketing services. If the order is referred via the restaurant's website, the commission is lower. Put simply, it's to GrubHub's benefit, and the restaurant's detriment, to try and promote the "microsite" over the restaurant's real website in search listings (whether they do or not, I don't know; that hasn't been reported on). They also list phone numbers on the microsites that aren't for the restaurant, but instead, for GrubHub's own services. GrubHub gets its commission, rather than being cut out of the loop. One could mount a defense for that (legally; ethically, there are...some issues with it) when it's done in the GrubHub apps; it's a bit stickier of an issue when it's done on a "microsite" masquerading as the restaurant's site and operated "on its behalf." As a side note, they've been sued for charging restaurants for calls that didn't result in orders, but that's a separate, albeit related, issue.
That would appear to go well beyond even the most expansive definition of operating a microsite. Even if you agree in principle to the idea, the poorly worded agreement means that both parties are likely to have very different understandings of what a microsite is and how it will be instituted.
0. https://www.theverge.com/2019/7/2/20680000/grubhub-seamless-...
1. https://newfoodeconomy.org/grubhub-domain-purchases-thousand...
2. https://thetakeout.com/grubhub-lawsuit-restaurants-phone-cal...