Well, 'more than three inches below the top of the hips exposing the skin' would imply showing one's butt, which would be covered under 'public indecency,' so it is a bit more than just looking like a douche.
so then this law is totally pointless, if its so low its public indecency do them for that
as for 7-11 rules well thats a private business, if your in their shop they can tell you what to wear, if you dont like it you can just go to a different shop