only certain parts are restricted, and what to stop them? you have to declare what you bought, if the officer decides to check certain things he/she is allowed.
if you do not declare an item you are smuggling it and committing a crime
as for whats stop us? the fact that if we get caught we're fucked.
"You know what the chain of command is? It's the chain I go get and beat you with 'til ya understand who's in ruttin' command here."