True BDCs are built for one specific ammo type, twist rate and barrel length. To put a BDC on issued scopes would either assume that the military only uses one singular type of ammo (weight, manufacturer, bullet type, burn rate, etc), twist rate and barrel length on all weapons...
...or...it would assume that things are "relative", "close", or "good enough", that no one will really know the difference. Or if they do notice the difference, they assume their "gun" shoots a certain way on the range, and then they'll translate that to combat. Even if they use a similar (but different) ammo in combat. They either don't know any better, don't much care, or aren't good enough that it makes much of a difference.
BDCs for soldiers ARE training wheels.
I do agree that magnifiers on red dots or holographic sights are crap, though.