In reality all the caches we place are on property that is owned or controlled by someone. So I guess we should have to get permission to place any cache except ones on our own peoperty and sometimes I am not too sure about that.
I figure if its not against any laws and is on public property then it should be ok. As for using it for a purpose not intended, what about Clark Kent using them to change into Superman. Maybe its like my old boss used to say "sometimes it is better to beg forgiveness then to ask permission"
Common sense should rule when deciding to ask permission or not.