I work in AK and "vacation" in HI, if I "lived" in HI I would be required to pay HI taxes.

Anyway, a lot of this whole state tax discussion has to do with domiciles and the actions you take to maintain/establish your domiciles (not in the Airline sense, in the Legal sense), if you were to work in CA for instance, brought/bought a car there, registered to vote there, established a bank account there and surrendered your license or a CA license a case could be made that you intend for CA to be your lawful home, however, if you have the aforementioned items in IL (or certain items in CA for convenience, but maintain your "Domicile" as IL), you could get around the law by making the case that you are not establishing a domicile, since you never "vacated" your previous domicile of IL, personally I like AK as a "residence" since there is no income tax whatsoever. The concepts of 50% of work are actually unimportant, if someone were to be from IL and went to live in Japan to work for JAL, but never vacated IL as their domicile and did not intend on making Japan their home then if the state of IL could prove that was the case they would be within their right to claim back taxes from you for earning IN ANOTHER COUNTRY.