What makes a 'federal facility' any more sacred than an office people or shopping mall or restaurant or movie theater or . . . .
Do you think that terrorists are merely seeking to target the government where there is some perceived level of scrutiny (not security) instead of unguarded targets with lots of civilians?
Your panic is not good reason to deny me my liberty without probable cause. There is no off switch on the Constitution.
It is not more sacred, but the government has a specific statutory authority to control security and require compliance at government facilities. This has been upheld in court, time and again. There is no Constitutional issue here.