A recent employment want ad for a manager listed one of the key duties as "to prevent union activity." How can this even be remotely legal?

I am not a labor lawyer, but here is my understanding of
how this could be legal:

Unions do have a right to be
protected from what are called "unfair labor practices."  These unfair practices involve
things like trying to prevent union activities or union formation through
coercion.  The want ad, as you have quoted it here, does not explicitly
call for any such coercive actions.

A manager could well
"prevent" union activity through non-coercive means.  Unions typically seem most
attractive to workers who are poorly treated by their management.  The want ad could,
conceivably, be asking for a manager who will act in ways that keep workers relatively
happy.  If the manager did that, unions might have a harder time gaining traction among
the employees.  Alternatively, the manager could try to persuade the employees that
unionization was not in their best interest.  If the manager does this without
threatening any punishment, there would not be a legal
issue.

I agree that this does not sound legal and was
probably not the most intelligent thing to put in a want ad.  However, "preventing union
activity" and "preventing union activity through coercion" are not necessarily the same
thing.