Being from outside the US, I'd never heard this term before, and actually in my country it's rare that you join a company and aren't given all the documentation by HR about what unions you can join on the first day.
But anyway, when I googled this term, from the wikipedia article:
> The tactic is often discussed in the United States because under US law unions may be prohibited from talking with workers in the workplace and salting is one of the few legal strategies that allow union organizers to talk with workers.
It'd seem them that at least one reason why they might explicitly protect the right for union members to lie about their employment history when trying to join a company for the explicit reason of salting is that they would also be / have been an employee of a union, and disclosing that could well get them deselected from consideration for the role.
Presumably the company will still be responsible for their own due diligence in checking that the potential hire had all the necessary qualifications to legally carry out the work, and might well discover the lie in that process. Presumably they could then also terminate the employee for that reason at that point because they couldn't actually legally carry out the job duties, but equally I'd imagine if they uncovered a lie which could be shown (presumably in court) to be for the purpose of salting, but they were otherwise legally able to perform the work, then the company couldn't fire them.