Again, forged documents won't cover that. Next, the people on this bus are passing through the campus, not necessarily employees getting out of the bus. Employees will have their ID's check as they enter their buildings.
And before the bus enters the federal complex. No unauthorized personnel are allowed to roam the grounds.
Your forged document argument comes under the heading "Told you these terrorists were stupid". The security detail has been trained to spot forgeries of all kind, spotting a fake ID is like a terrorist in the bank. What could be a bigger admission of guilt than carrying a falsified ID into a secured federal complex. That is exactly what they are looking for and that is one good reason to check IDs. Oh and the terrorists would not bother to attempt a fake if they didn't believe that IDs were being checked faithfully. Another good reason to diligently check legit IDs.