This is an age old problem. There doesn't have to be a rule for there to be a rules violation ! The rule is that US carriers and carriers flying to/from the US are required to have a safety & security policy. Once they have such a policy, they violate the rules by not enforcing it.

As an example, there is no particular FAA rule dealing with carry-on luggage. But, if a carrier limits a pax to X number of pieces of Y dimensions and fails to enforce that, the carrier and personnel are subject to penalties. While it is technically incorrect that the third carry-on "violates the rules," the practical result is the same.

If a carrier's policy is "no movement between cabins" it's a violation not to enforce it.

That's why the ongoing back-and-forth with DHS is meaningless.