Several reasons why they didn't open fire....
Wow--I'm surprised the tower guards didn't open fire.
1 It was dark. Hard to see cons climbing up the rope
2. A helicopter coming into a prison is almost always the good guys & thats probably what they thought too.
3. You don't shoot at an aircraft not knowing who is on board. Good thing they didn't. Had they done so they'd have killed an innocent pilot who was hijacked and being held at gunpoint.
4. A helicopter crash would have jeopardized innocent people on the ground.