The mag disconnect safety is an option for the same reason the safety lever is, they are trying to market the gun to anyone and everyone. That was never a decision based on making the gun as safe as possible, it was decision based on selling as many guns as possible.

Believe it or not before I was a cop I was a civilian with a CCW (not for long, but I was). The first thing I did when I bought my S&W 915 was to enroll in a handgun course taught by a former Secret Service Agent and FI. Then I started carrying. I was not new to guns at all, but I was new to carry a gun concealed and using a handgun as a tool for self defense. My home state requires NOTHING to get a CCW, but that class was packed full of people. Some new to guns and some not so new.

You don't have to take the motorcycle safety course to drive a bike, but you and everyone else on the road are much better off if you do. Same thing with handguns. A lack of training equates to ignorance, ignorance equates to negligence, negligence gets people hurt or killed.