For me, it WASN'T greyed out last night and I changed it to "off" and watched expecting to see the time at the top flick to 12hour, it didn't.
Got sidetracked with other stuff, looked in settings to try it again this morning and it was greyed out with "ON" stuck in there.
I don't remember, in fact pretty sure I didn't, changing the region or international settings. Perhaps it auto set on one of the syncs or something.
annoying though, hope there's a simple answer other than changing it to be US
Exactly the same thing here - button available so I tried turning it off then went to back home screen and straight back to check it and it was turned on and disabled! sounds like a bug with Apple's disabling code: Why they'd want to do it is anyone's guess though. Fortunately, I always use the 24 hour clock so it doesn't bother me, but it's still unnecessary and a bit buggy on Apple's part