1. If this was an international flight, movement between cabins is prohibited by DHS, not FAA (DOT). And, that rule permits cabin crew broad discretion and that broad discretion results in the rule largely being honored in the breach, the smart thing being to consult with a crew member before invading hallowed ground.
2. On domestic flights there is no such rule. Air carriers are required to have a safety & security policy and to file that with FAA (DOT). Once filed with DOT, the failure to enforce the policy becomes a violation. It is a distinction without a meaning.
3. The stated reasons for requiring use of lavs in ticketed cabin are safety & security, not DYKWIA. I take it that the aircraft landed safely?