Exactly mates rates. What is stopping a car/truck driving along QF Drive, stopping as an aircraft is on final approach and stopping to denoate something?
Terrorists are always going to get around any rules in place. The only way to stop it is stop all flying - but the then the terrorists win.
It is the same as car accidents - governments keep putting in rules and punishing people, but while there are roads and people drive, there will always be accidents and people will be killed.