This is not my area of expertise, but, of course, I have an opinion.
When you blow up a toy balloon, the pressure inside is only barely more than the pressure outside. The difference in pressure forces the elastic material to stretch until the forces are balanced.
The skin of an airplane is similar to a balloon, but is on a frame so that there is a pre-determined shape when the pressure is equalized by opening the doors. Also, the material requires great force to stretch it, so it doesn't expand very much at altitude.
At cruising altitude, the small pressure difference creates tension in the skin of the plane, but the shape is constrained by the airframe.
Now imagine what happens when you stick a pin in a toy balloon. The tension which existed in the stretched material is disturbed and the material begins to tear. The balloon "pops" because the tearing is very quick and the pressure is allowed to equalize very quickly.
If a bullet passes through the skin of an airplane, the tension in the skin of the airplane is disturbed and it is possible (perhaps likely) that the skin will begin to tear. The change in forces is so rapid, that substantial damage can be done to the skin before the pressures inside and outside the airplane are equalized.
The solution to the problem of catastrophic damage caused by a bullet hole in the skin of an airplane is to design the framework to which the skin is attached, or possibly the shape of the skin itself, so that the tearing which takes place after a hole is made is limited during the decompression. Everyone would have to reach for an oxygen mask at cruising altitude, and perhaps one or two people might find themselves outside the plane. (Note: don't let your hamster play near an operating vacuum cleaner!)
The details and cost of making the skin of an airplane "bullet-proof" in this fashion is complicated enough that I am still working on an opinion.
The important point is that there are things which might be done to allow passengers to protect themselves. Making them defenseless is proven to cost thousands of lives.
Contemplate also the UA 747 that had a major chunk of its side ripped off leaving Hawaii back around 1989 due to a bad latch on a cargo door (I'm not referring to the 737 that developed a sun-roof).
Several passengers were sucked out because their seats were ripped from the floor, but overall the aircraft returned to Hawaii and landed safely.