Um, have you actually heard the really stupid reasons why people won't buy Macs? Many of them are for even dumber reasons than that.
You can argue that those kind of people might not buy a Mac anyway, finding any excuse. But then again, with some people, it really only takes that ONE THING to turn them off. So why give them more ammunition? Is it petty to be turned off by one little thing like that? Sure. But the point is, some people ARE that petty. And at the end of the day, the end result can still be a lost sale. And while it's true that person might be an idiot to give up on a Mac due to something as lame as having their Facebook banned, Apple is being JUST as silly for not thinking of a BETTER solution to handle the computer-hogging problem.
Why BAN a harmless website, when there are so many other simple alternatives you can do that will ALSO limit people's time, but not turn anyone off to your product? It just doesn't make sense to me...
Maybe instead of just restricting access to the site, they could redirect the person to a page stating that the site is only banned because it causes unnecessary congestion and loitering in the store.