Why should they be restricted? They are exclusively defensive devices.
Because that way the Imperium maintains military superiority over its citizens.
Meson screens and dampers frustrate Meson Guns and Nuclear Missiles. Ships without them, even at the same tech level, are at a severe disadvantage against Imperial ships, which DO mount Meson weapons and nuclear missiles.
What effective precludes them in most comercial ships is its price and volume and power requirments. Unless you really expect them to be needed (so, that you can confront nukes or messons), why to spend so much money on them?
Right, so banning them isn't an issue, is it?