No, I'm implying that working for an actor’s counterparty compromises your ability to represent that actor in adversarial interactions with the same counterparty, which is a core function of unions with respect to employers as it is with defense attorneys with respect to prosecutors.
> companies certainly have the option to hire employees that do not work for unions.
Not in the US; even where union shops are prohibited by state right-to-work laws, adverse employment decisions on the basis of union membership or union-related activity by an employer are prohibited by the National Labor Relations Act. That is, in some cases, employers are allowed to hire employees who aren't members of a union, but they aren't legally permitted to hire employees because (even in part) they aren't members of unions.