It's security theater because the entire thing is done under the implication of protecting us from terrorism, when it does no such thing.
Yes, yes, a few people have been prevented from bringing guns in - but there is no evidence they were intending to do anything with them. It's just those idiots who think they have to carry one everywhere. I don't want them in there, either, and I think those people are sad, fearful souls, but there is no evidence anyone was protected from anything.
If Disney were actually serious they would be at the very least swabbing, if not thoroughly inspecting, every stroller brought in. You could get an arsenal in there with some of those SUV sized things.
That said, as we just learned from Vegas - the front gate of the parks is not where the security really needs to be worried about if they are trying to prevent some mass shooting.