Enforcing it is free: you can send your own cease and desist for free. The only time you need a lawyer is if they refuse and you want to sue them. If the court rules in your favor, you can even sue them for the legal costs and costs of damages.
In the end it's less work for the investor-backed business, since with the C&D route they would need to register domains or create pages for restaurants that don't want the service only for their (very expensive!) legal department to receive a letter so they can delete everything.
I mean just send an email to the restaurant so they can submit an authorization beforehand... tell them the advantages, let them make the decision...
Enforcing it is free if your time is free.
But suppose we indulge your fantasy. A restaurant owner, who is already incredibly busy, decides to figure out how to register a trademark and sends the cease and desist. The venture-backed startup's in-house counsel looks at it, sees that it was written by an amateur, and just laughs. Now what?
(And as I understand it, you get some trademark rights just by establishing a business, so you don't have to register the trademark to sue UberEats or Grubhub or whoever)
Are you proposing to do the work of writing up and sending C&Ds for these restaurants personally gratis? That's the only way it's "free".