a) AC91-57 has been around for decades without AMA lobbying to get it changed.
b) ASF400 talks about what happens to UAV (the things we fly) that dont stay within AC91-57.
c) Guidence0801 is the decendent of ASF400, and it too talks about what happens to folks that dont stay within AC91-57
d) There is also a Policy Statement from the FAA to clarify it.... and it says the authority to fly models is AC91-57.

Now, with all that redundancy over decades of updates, we are still being pointed to AC91-57 as what the FAA says we are supposed to do. But even with all that, folks still claim they are abiding the 'voluntary' nature of the AC by simply not volunteering to comply.

Consider this analagy:
Rule 1: 400ft is voluntary
Rule 2: You will be punished if you do not obey Rule1

But rule2 never changes the voluntary nature of rule1,
and folks are exploiting that to say they are obeying while choosing not to volunteer to 400