That's mistake from the people that updated the spec after they ran out of amendments, as they obviously did not talk to the ammo guys. 5.56mm and 7.62mm EPR rounds have a copper fouling issue in the barrel (and accelerated barrel wear), which even though there is a de-coppering agent in the propellant, it's still not enough to mitigate the copper fouling. It's even worse with tracers.
And it's going to get worse with the Army's Green Primer initiative to make EPR rounds fully green, along with Next Gen Ammo.
lead has a magical property to mitigate copper fouling.