The ban is an reaction to a creative way of using Airdrop as a not mass-censored communication tool with strangers, by pure chance of them having Airdrop open. This method was know for some time, but has been more extensively exploited following the Beijing Sitong Bridge protest[1] and during the course of 20th National Congress meetings, even though this is extremely inefficient, unsafe in terms of anti-forensic, and only available to a rather small group demographically.
The user base of the above method is extremely small, yet retaliated by their schools or police apparatus. Now they want to make sure that this communication channel to not exist.
[1] https://en.wikipedia.org/wiki/Beijing_Sitong_Bridge_protest