USAF NAVY plane at RIAT, why the security?

Hi,

anyone that went to Fairford will know the aircraft type I am about to describe (stoopidly I failed to get it’s type).
At the far west end of the runway was a USAF NAVY 757? that had it’s very annoying APU running 🙂
I’m curious. Why the armed guard? Or is that standard practice for American aircraft?
Was wondering if it had sensitive equipment on board?

Regards,

Hampden98