I think that's an over-simplification.
To put a statist spin on it, people do not need to be experts on cars to drive a car but they do need a driving license, people do not need to be experts on gas boilers to install a gas boiler but they do need to be certificated to install them - so perhaps actors required to fire guns should be licensed also, or at least trained to a sufficient standard.
To me, the two major poles of this argument seem to be that a) the armourer bears sole responsibility (Jim's position, I think - I don't want to put words in your mouth so please accept my apologies if I've misunderstood) and b) armourer and actor share the responsibility, which is my position.
There is no need to go straight to the extreme of demanding that everyone become an expert at everything they intend to do but, especially when another life is in potential jeopardy (in this case the actor who was shot), some level of proficiency would seem to be in order. The argument "how was he supposed to know, he's only an actor?" holds no water for me. I'm a campsite handyman and gardener's assistant with no special training or in-depth knowledge but I know that guns are dangerous, even toy guns, and should never be taken lightly.