I suspect several reasons. Mostly because they are "aligned" with him. It's also a fact that Hollywood is a big cabal. If you wanna work in that industry, you tow the line.
As a whole they are so damned ignorant of guns. They don't know how they work, what they actually do or how they do it, much less how to handle them properly. They know what they've seen on TV.
SAG got a lot more serious after Brandon Lee (Bruce Lee's son) was killed. His death was clearly a screw up of epic proportions but totally understandable when you consider the ignorance factor. Now they check EVERYTHING and then check it again. Ammo, gun loaded condition, gun mechanical condition and on and on. They rehearse and dry run scenes so that everybody involved knows what is to happen. The place cameras so that guns don't have to be actually pointing at a person but still appear to be. They hand a gun to the actor and show it's condition to the actors (plural) and if it's a "cold gun" they will dry fire it so all involved know that it is actually a "cold gun".
I can't even fathom how long or the number of people involved in the big shootout scene in Heat if done per the norm! Jeeze!
