I think the complaint here is not that airline are restricting availability (of course they can), but rather that there is no way for people to see what the real availability is (apart from checking specific dates one by one).
And sometimes it is so limited (and/or non-transparent) that it defeats the purpose of certain fares, e.g. AONEs where you end up flying in D (or lower) half of the time because you can't find availability in A.