[Centurion2000]

They do the exact same thing here in Texas as well. No guns in federal or state buildings and courthouses.

Possible rationale: too easy to engineer a breakout.

[Marauder]

For courthouses, there's a more likely rationale: When your everyday work involves working for one side of a conflict, or deciding against one side of that conflict, you would or should fear being a target of someone's wrath, which might and in several cases has involved a firearm.
I used to work at a law firm as a file clerk back before the Texas carry law was passed. Every lawyer was a staunch democRat and pro gun control, but every one of them kept a pistol or revolver in his desk.
Later, some judge was shot and killed in his courtroom, and that's when the metal detectors went up in courthouses almost immediately.