It's just speculation on my end but the way that the firearms have been removed mid event in the past always made me feel like it was a directive coming from higher in the Universal food chain than from the Horror Nights team. Some executive sees a clip and shuts it down.
Also worth noting that the concept art for Midsummer Scream showed what was clearly a World War 2 era Anti-Aircraft emplacement, while what we actually ended up with with much more of a gun-gun. Still just speculating, but they may have been approved for one but not the other.
Regardless, its a shame that it's gone now, having the plane crash be part of the maze's story instead of just randomly having a crashed plane off to your left was a big plus for the tram this year in my book.