Not just a S&W thing, regulations promulgated by the Attorney General's office (940 CMR 16). Here is the relevant bit from those regulations:
It shall be an unfair or deceptive practice for a handgun-purveyor to transfer or offer to transfer to any customer located within the Commonwealth any handgun which does not contain a mechanism which effectively precludes an average five year old child from operating the handgun when it is ready to fire; such mechanisms shall include, but are not limited to: raising trigger resistance to at least a ten pound pull, altering the firing mechanism so that an average five year old child's hands are too small to operate the handgun, or requiring a series of multiple motions in order to fire the handgun.