Sadly, it is still not clear what law applies when a plane is in the air (there has not been a litigated case yet) so it is not necessarily safe to assume a particular state law is in force unless the plane is on the ground. In the now famous (and still on-going) Gillette case against Delta airlines in Vermont, Delta (unsuccessfully) argued that state law did not apply because airlines are federally regulated. However Gillette’s success on that point was tied to the fact that the plane was on the ground and therefore clearly in the state.

FWIW, I think http://llli.org/Law/LawBills.html is a better source for state breastfeeding laws because many of the links on the National Council of State Legislatures site you link to are dead (they link to the bill in the legislature which usually disappears once the law is moved into the state statutes). Knowing the state laws is important for the time you are on the ground, either in the air or in the airport changing planes or during layovers. Also keep in mind that in states without any enforcement provision or penalty in the law, there may be little you can do even if the law is violated. (See http://www.mothering.com/articles/new_baby/breastfeeding/lactation-law.html )

I am not aware of any airline that has a written formal policy concerning breastfeeding on their airplanes. Unfortunately, one hears of negative experiences with flight attendants all too frequently. This is very serious but is still, I think, the exception and not the rule.

If you have a news article that quotes United as stating that company policy allows breastfeeding, I suggest printing that news article to show to any United personnel that might give you a hard time.

Hope that helps.