Why Many Americans Don’t Understand Machine Gun Laws

This sounds like a simple answer to me. In my opinion, Americans don't understand machine gun laws because they don't care about learning. Others, haven't given it a thought. While others are confused about the difference between fully automatic and semi automatic. This is an interesting article about both.

It’s totally legal to own a machine gun, but the laws and regulations are widely misunderstood.
The instructor’s face wore a frown, and I was worried that I’d committed some safety violation that was going to get me kicked out of the rifle range. When you’re firing the same kind of machine gun that killed Osama bin Laden, a weapon that can spit out 900 bullets a minute, the rules are many, and strictly observed.