Simple Explanation:
Most users are too stupid to stop tightening the screw and you will strip out the threads.
Extended explanation:
The average gun owner is border line legally retarded. They don't comprehend simple concepts of material strengths and so on. Just look around this message board and see some of the problems people post about and questions asked. You will quickly realize that most people who own guns are lucky they figured out how to put their pants on in the morning.
This is especially true with modern military rifles. Most mouth breathers that buy guns so they can be the super awesome envy of all their friends will buy the latest and greatest evil black rifle to pose with. As such, most users will over tighten the tensioning screw and strip out the threads in the aluminum base.
Is this an over-generalization? Yes. Is it unfounded? Hardly. Working in the industry and spending time on this board has shown me everything from users tightening the bolt enough to bend their rear sight (I s**t you not), to people who don't know which way to turn a simple nut to remove a muzzle brake.