Both a EWR Redcoat and TSA told me the advance time was needed to security-screen those seeking to gain admittance and prevent last-minute people from trying to get through without proper security clearance.

Whether this is true or not, who knows... but the few times I wanted to use the PC when not flying, they wanted the 24+ hour time to get a pass together.

It sounds like its easier to buy a refundable ticket and have it refunded when you arrive in the PC.