I've never had to sign a contract to eat in a restaurant. Are you referring to a specific restaurant?

I think they just mean that in many restaurants you pay only after receiving your food, but it's really not a very good comparison for a number of reasons.

Especially since all of the above listed things, but especially restaurants, have stated prices and often a means of negotiating them down or being entirely un-obligated if the providing party fails too severely.

And since, though not fully related, a restaurant can choose not to accept credit cards for instance, but there's often an obligation to state that clearly and visibly somewhere obvious beforehand, and always (in the US) an obligation to accept cash for debts owed in this manner.

Service charge? Mandatory tip?

Tip is never mandatory, a service charge has to be clear up front to be valid.

Contracts don't necessarily need a signature to be valid. If you order food at a restaurant, there is an implied contract that you will pay for the meal.

However, this amount is almost always stated in the menu, so it isn't arbitrary. In fact, in each of the above cases, the amount you pay is almost never arbitrary, but based on usage or a preset price.

I am allowed to cancel my cable service and cell phone plan. You can't cancel a timeshare.

But you still have no idea how much you'll actually pay when you sign up. Even if you can cancel it later.