If you ask me
It seems to me that the subject of risk assumption should be much simpler to suss out. The job of a regulator is to tell you what you may do, under the certificates you have been issued. Their follow-up requirements are to ensure you are operating pursuant to said certificates. The thing that is a keystone here is that their regulations tell you, as stated, what you MAY do. They can never be expected to tell you all you may not do as the entirety of that spectrum would dwarf a normal library. So as long as passengers have approved seatbelts and have been briefed and are wearing them as required, what happens beyond that is the responsibility of the owner operator and pilot. The FAA may have a hard time banning restraints that are not required in the first place. What they could do is ban the flight with doors open or off by putting it in the GOM manual for the company.