If it's so vital that non-licensed users don't overhear ATC frequencies, why not use digitally encripted radio similar to that used by GSM/TETRA (I'm referring to the encryption, not the mobile system itself)?
OK so the answer is cost and compatibility, in which case, why the fuss about people listening to a broadcast on an un-encrypted network, so long as the equipment used by listeners does not degrade the service?