If you want a bit more rationale:
- Compliance is an easier process if you designate responsibilities. In most cases, compliance is split neatly in two: the SM is responsible for keeping track of rules concerning hours of work, acceptable duties, acceptable working conditions, etc.; and the PM is responsible for rules concerning construction safety, effect safety, and other things in that vein. There is some overlap here (if the SM sees an unsafe element of the set, they have the authority to follow up on it), but having designated roles means you avoid the "Bob thought Jim was going to take care of it, and Jim thought Bob..." scenario.
- The PM has a phone line and an office and a desk and office hours and the time and resources necessary to draw up contracts, consult with authorities, disburse funds, and so on. The SM doesn't, at least not without running everything through production management anyway.