In my friend's case, it's because people in the neighborhood sometimes like to pick things up. Why should they penalize those people just because delivery companies have without their permission decided to start advertising their food?

Agreed, and this reminds me of friend who started to carry foldable chairs and table in their car during lockdown. They would, sometimes, set those up in the parking lot of the restaurant and eat there.

However, perhaps a solution might be a labeler that prints a label with consume by time and attach the label to carryout container. Or some other kind of disclaimer on the containers. Yes, this adds burden on the restaurants but hopefully, reduces negative reviews.

Or we could place the burden correctly on the delivery company, and require them to work with the restaurant to create the optimal delivery experience. Say, by mandating that delivery companies actually have to get a restaurant's agreement before advertising their food.

This is removing to-go option. There are apps that claim to offer delivery from a restaurant not intending to provide it.

Legally the only way to acquire restaurant food over here atm is takeout or delivery. They can't just not offer it.

so the remedy is to implement a law that will live forever, even as we expect covid to (hopefully) recede this year?

This specific issue was an issue pre-pandemic as well. The surge in take-out has amplified the problem.