The lower especially the design of the trigger, trigger sear and the safety sear is what makes it able to go FA.
The FAL design is FA but has been made to SA only and yes FA is way too easy. The upper portion is just a block of steel with a barrel and a gas tube. Nothing special there. I do believe that most firearms are categorized by the lower, frame or where the fire control is. Plus with the overall design the magazine is too far forward to be comfortable. The FAL design just fits into your arms with the mag against your inner forearm for support.
My 2 cents.