I assume $10 per domain and GrubHub is shown to register tens of domains. Assuming a popular restaurant is worth 50 domains, they only need to capture 20% of average $25/order for qty.100 orders through 50 domains in 1 year to pay the annual registration cost of the scam.
The scam is that restaurants can’t as easily pay $500/year to prevent GrubHub from squatting on and SEOing typos of their domain, and GrubHub can simply go make up another 50 domains when the owner takes over the first 50, because they only need the 20% “marketing” ripoff from 100 orders to break even again.
Now, what Grubhub is doing is unethical as fuck--best case. Worst case, it's criminal impersonation or business identity theft.