They take a bit off muzzle velocity and reduce the penetrating power of those little bitty 5.56 even more.
Why do you think so?
My reading, is, if anything, they increase muzzle velocity a small amount.
Essentially, the act much like a barrel extension, except compressed into a shorter length.
So not only does this move protect enemy ears but lowers enemy penetration. The Marines getting adverse to harming enemies must be part of some as yet unrevealed social experiment. The Kumbaya Experiment?