I don't know about this matter in the US, but it seems to me there is no need for law a regulation to revoke a visa arbitrarily. There are countless reports of

I don't know about this matter in the US, but it seems to me there is no need for law a regulation to revoke a visa arbitrarily. There are countless reports of this kind of thing happening at the border, some have even been confronted to screenshots of their private facebook messaging.