They also say:
There are also different implications and requirements for union recognition. For example, in the UK, staff members can individually decide whether to join the union or not, whereas in the United States all union eligible staff would be covered by a collective bargaining agreement, and in many states it is possible that all union eligible staff may be required to pay union fees, even if they choose not to participate in signing cards or voting.
That feels like it couldn't possibly be true, but if it is that honestly seems like a fairly good justification for letting the union process play out without managerial interference.