I would think a company as large as an airline would have cleared such a thing with their own lawyers, before providing fodder for other people's lawyers in the form of a publicly accessible website. The point is that they made an effort to notify potential customers of their policies, who can then decide whether or not to purchase a ticket from them.
All airlines have a "Contract of Carriage" that you agree to abide by when you purchase a ticket. Most of us don't bother to read it before happily handing over our credit card numbers, but it's there and we are obligated to abide to the parts that apply to us, just like they are obligated to abide to the parts that apply to them. This policy is simply an extension of that contract.
A few people, albeit none of them lawyers, have already stated that you probably don't have a legal basis to go after the airline. But, this is America, and you are welcome to further tie up our already overburdened court system over yet another trivial matter. Most people who are unhappy with Hawaiian Airlines' new policy would simply take their travel dollars elsewhere.