I assume you were using a weapon that cannot be zeroed as some weapons have no possibility to do so. Instead their default optics usually have markings to help calculate correct distance and elevation in which to fire at the enemy.

Negative. The MXM, Katiba, and the MX 6.5 are all able to be ranged when an optic is not attached. Those are the only weapons I've tested with, but they can definitely be ranged — only without a sight attached.

Well the reason for this i think is because the scopes already have range allocations on them, center being 300 one down being 400. so changing the zeroing would mess up the ranges on the scope, its an odd way of doing it though.

Not a bug, working as intended.

See #3540.