"allows existing AAD users in your organization [..] to initiate a trial of the product."
Because we are going to ram this baby down your throat whether you like it or not. And that is why we activated this ability before giving you the possibility of locking it down.
Possibility which will unfortunately have a hiccup in a future patch Tuesday, unlocking user access once again and taking into account ever user who asked and was blocked by the setting that will unfortunately be reset.
Because we can.