Why is McDonald's forbidden to eat bring your own food?

This is not a legal requirement, but an industry norm.

I remember that a few years ago, hotels and restaurants in the catering industry jointly signed an industry standard, which included the content of "No Bring Your own Food", but it was not enforced, but was stipulated by hotels according to their actual situation. Actually, it's the same as nothing.

Generally speaking, it is illegal to do so, but there is no relevant provision in the law, which has been exploited by the hotel.