Let's see. Granny drives up in a rental truck. Granny rides the bus. Nah, no different in threat potential. Granny can sure pack a lot of fertilizer in her purse.
Some of you guys really crack me up. You demand a wall around the USA so Jose Illegal can't get a job cleaning toilets...but free and unfettered access to federal properties.
So your solution is to do nothing, because everything can't be done? Sounds like the anti-war left complaining that all the money being spent in Iraq should be spent here instead. Well, was it spent here before the war, and if so what good has it done? Doesn't work that way.
We try to deter granny with every means possible whether a rental truck or a bus. Doesn't mean it's 100%. Doesn't have to be 100%.