It’s laziness. Automatics are modified in a way that prevents them from being fully operational as a gun, but not for safety reasons.
They won’t cycle with blank rounds because there’s no backpressure from firing a live round, so they obstruct the barrel to redirect some of the gasses back into the action.
For revolvers, bolt guns, etc that isn’t an issue because they aren’t cycled by recoil or gasses. You can just load a blank and use it.