I'm not sure why it doesn't make any sense to you. The trigger mechanism on the AR-15 is designed as such that it can only be placed on safe if the hammer is cocked. In fact just about all firearms that use a trigger/hammer block safety (and the AR-15 is one of them) operate this way.
Just about the only firearms that I can think of that you can put on safe while the firearm is not cocked are pistols with slide mounted safeties.
The point is, the fire control system on an AR-15 (which is what the M&P15-22 uses) is not designed to do that. So either the fire control unit or the lower receiver is out of spec. If the specific part that is out of spec happens to be the safety or the trigger, it's possible that the safety lug on the trigger is not being held down far enough to adequately prevent the sear from disengaging.
The concern is also compounded by the fact that the safety selector switch is made of a polymer material. Since the fire control unit is out of spec enough to allow the safety to be engaged, but not without resistance, which could cause wear to the safety selector. This wear could also lead to a unsafe condition over time.
The other obvious concern would be possible damage to the rifle by someone trying to cock the rifle (from an un-cocked condition) if the safety is already on. The geometry of the fire control group does not allow this. If forced, it could lead to damage of any of the following: fire control group, lower receiver, charging handle, bolt carrier group and there's even a remote possibility of damage to the upper receiver.
The bottom line is, if your firearm (any firearm) operates in a way that it's not designed to, it's malfunctioning and possibly unsafe. Don't ignore it. Don't screw around with it, get it fixed.
WOW, imagine the discount destruction you could lay down with that!! Too bad its $600 bucks but lordy think of the fun you could have, pieces of little squirrels everywhere.
