State law or contract language cannot violate US Law. In that case they are unenforceable.

Thirteenth Amendment:

Section 1. Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.

When you sign the contract for your training an indenture is created for the period of time specified in the contract. This kind of contract was made illegal by the Thirteenth Amendment. That is why no airline has ever tried to take someone to court over this because they have good attorneys that know it is ultimately unenforceable.

So, moral issues aside, they can't force repayment through legal means. Now, they can report it as a bad debt, they can constantly harass you according to the Fair Credit Reporting Act, and future employers may look unfavorably on it if they pull your credit report or you tell them what you did. But, they can't force you to pay it back.