If the company actually sues you, these types of cases are easy to get a lawyer for on contingency for the counter-suit.
It is a tort to cause somebody to be unable to work. Trying to ban them from the industry they have experience in, when they're not a worker with specialized knowledge about the company's professional whatthewhat, is a giant counter-suit.
And, did they give these workers a pay raise when they added the clause, that they can argue in court was worth 18 months of work? No? Then it isn't enforceable in any state, because of the basic common-law requirement for contracts to be based on a "consideration" given. If they argue some tiny amount counts as consideration, they'll be admitting to an unfair business practice that denies somebody the right to work.
They wouldn't just lose, they'd get shafted on the counter-suit. Except that they would actually just pay lots of money and settle.
This is a clause added entirely because Amazon's lawyers are assholes. Luckily, asshole lawyers don't get to file suits without the permission of the senior execs.
I predict somebody will quit, and then sue them for 18 months of pay because they're only experienced in warehouse work. They'll not only get an easy settlement, (these contracts are only legal for professional employees, not for laborers) they'll also throw it out for everybody else. They'll still probably have to pay settlements to a bunch of people who quit or were fired during the policy, and then didn't find work immediately afterwards.
They should really hire some competent lawyers instead of these "full asshole" guys, because they're creating a giant pile of needless liability for nothing.