Yes, this is an interesting development. Two years ago Congress passed explicit Federal legislation governing carry on musical instruments. The law was pretty clear that if the musician carried on an instrument and room in the overhead was available at the time of boarding, the airline was required to allow the carry on. Seems simple, yes?
However, from what I am hearing, the airlines seem to have found a loophole; and I suspect it is some kind of Homeland Security based nonsense or something, as though your bass is a threat to the airline or something. The reason the airlines do not want the instruments on board is that they are trying to maximize the number of rollerboards that go in the overhead. This is because, with the exception of Southwest, most airlines are maximizing profits by carrying freight, including dangerous chemical goods, in the baggage hold. So, they have a business interest in minimizing checked luggage. It seems the musician is the loser in the equation.