But it does make it legal, right and ethical according to the law. Not according to the MSC or according to me. Established commercial practice can be used as the basis, or part of the basis, of a decision when a contract is disputed. (Not my opinion, but the statements of the attorneys, who I assume know more about contract law than we do, other than the one or two attorneys here who also mystery shop.) That's why I hope a representative from an MSC will chime in here.
Mystery shopping is beholden to contract law; it's not exempt from it. Shoppers can't make up rules just 'cuz we don't like the law!
Another factor is if the act of someone else paying for the meal interferes with or makes it impossible for the shopper to accurately complete the shop. For example, if the shopper can't finish the timing observations.
I don't know the shop that was discussed last year, so I couldn't speak to the precise wording of the guidelines or send a redacted copy to the attorneys. As in all contract situations, the attorneys would need to see the written contract in order to form a complete opinion.
I learn something new every day, but not everyday!
I've learned to never trust spell-check or my phone's auto-fill feature.