I'm not so sure that clubs can't be held responsible for the conduct of their fans. If United get away with this scot-free (which I think they have done, because I don't think they have been charged), then their fans are given carte-blanche to do the same thing at any game they choose. In theory, they could disrupt the full season.
If The FA were to punish them with a points deduction, their fans might think twice about having games postponed in the future. United might start taking security a little more seriously, which would be good for all visiting clubs.
The problem at the moment is, as with so much at The FA, that their rules are poorly defined. As you point out, they have decided United had 'just cause' on this occasion. Yet Middleborough were given a points deduction when their squad was decimated with flu, and Spurs were forced to play when their squad was hit with a virus in 2017 (the lasagne was innocent).