Federal regulations are just as enforceable, by federal authorities.
Congress passed l;legislation giving them authority, so yes is based in law.
When get that hefty fine when crash into a crowd, forbid a plane, other issues, will think otherwise
Not saying Farmer John flying over his land looking at cattle never getting near 400AGL would have an issue, no one will ever know, then there are low flying crop dusting operations, opps.
I have a good bit of open property, and have all kinds of ultralights, para-gliders hot air balloons operating under 400AGL over me at times so having to be very careful and always VLOS, if not could easily collide with one of them, the the feds show up. So autonomous flying good only for emergency RTH flight but should never fly without VLOS.