There was a similar story and thread posted at Xmarks a while ago (a year, maybe?). What came to light in that thread is that in Great Britain, kilts can mean trouble, sort of like wearing your gang colors. It may well be that management just didn't want trouble.

There was another thread here about why more Scots don't wear the kilt casually. This, I think, helps explain that. They really do seem to relegate the kilt to formal/special occasions, and wearing it casually tends to break some sort of societal norm.

In the other thread, I opined that this is a bit of evidence showing that the Scots are forsaking their national heritage. I've since softened that position. But the next time you're out casually wearing the kilt and you happen to run into a Scot who tells you that you just can't do that, you may reply, "Neither, it seems, can you."