Pardon my ignorance - Which pistols require you to pull the trigger in order to field strip?
It's part of many striker fired designs, namely the Glock and S&W Sigma (which is mostly a copy of the Glock). Reason being, the striker is partially cocked by the motion of the slide. Pulling the trigger allows the striker to be completely uncocked, allowing takedown.
The S&W M&P has a little lever inside that allows the user to move the striker off the sear to avoid having to pull the trigger. Since you have to retract the slide to get to the lever, it forces a person to check the chamber and even stick your finger in there.