First; just because it's not explicitly allowed doesn't mean it's considered acceptable. I wouldn't want to live in a society where we have to put up signs and write explicit rules for every single aspect of our lives - we're social beings, we should be able to figure these things out as they arise. There aren't any "No spitting on the floor" signs in the library, because people can figure it out without the signs. Likewise, there is no "Shoes required" sign, because the overwhelming majority will be wearing shoes.

Second, calling security because someone isn't wearing shoes is of course a bit silly, but the thing is, bare feet freak some people out, big time. I haven't quite figured out exactly why, but they do, and when that happens, people will be unable to react rationally. Fear, panic, overreactions, and a complete absence of logical thought, are all very common, for better or worse, and you are not going to change that, at least not on your own, and not on the spot.

So I guess what I'm saying is pick your battles - you can be a dick about it, or treat it as an interesting insight into human psychology. And believe me, I totally understand your need to vent that anger.