Yeah, well after 5,000 years of fighting among those Semitic first cousins, there should be peace in the Middle East, but there isn't. All the FAR says is that if the airline promulgates a carryon bag rule, the airline must enforce it, and can be fined if they don't.
As a matter of fact, the TSA quite clearly stated that they are NOT responsible for enforcing rules set by the airlines or the FAA. Their charter from Congress does not give them carryon bag rule enforcement capability. They are strictly limited to "security" issues. Also, keep in mind that though they are both U.S. Government agencies, they are not in the same Department--TSA in Homeland Security; FAA in Transportation. Turf wars are common among Cabinet departments, and cooperation between those departments is not the first rule of the day.
Whether you or I think that they should enforce the rule or that carryon bags are a security issue, the TSA does not.