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.
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.