If a business is providing wifi to its customers, I regard it as reasonable that the service be paid for by showing ads to the customers, also it's reasonable that the users be expected to agree to an acceptable use policy.
That this is presently implemented as a man in the middle attack is a failing of the 802.11x protocols. the 802.11x standardization committees should have agreed on a documented, standard way for access points to do what so many had to bolt on as an afterthought anyway.
Most would regard inserting ads into third party websites as an attack. Moreover, there is no obvious reason why the acceptable use policy has to be delivered via MITM attack rather than e.g. posting it on the wall. It's not like you can expect anybody to read it anyway.
That this is presently implemented as a man in the middle attack is a failing of the 802.11x protocols. the 802.11x standardization committees should have agreed on a documented, standard way for access points to do what so many had to bolt on as an afterthought anyway.