Are the line crossers actually maintaining their membership in the union, thus subjecting themselves to potential discipline? Or are they sending in letters of resignation/reduction to "representational fee paying" status only, thus placing them beyond the bounds of any union discipline efforts? I would have thought that, even if they were just half-way knowledgeable, it would be the latter. While I believe that companies aren't allowed to actively encourage members to resign from the union, they are allowed to present the alternatives available in that direction if they so choose. I would have thought that Oak Harbor - given that it's talking about using permanent replacement workers - would have presented those alternatives by now. Haven't they?
Once a [past] member goes that route, it tends to put (to use a mixed metaphor) on the other foot; i.e. - if the union harasses or tries to intimidate the past member, or fail to represent him simply because he's no longer a member, it commits an unfair labor practice itself, and subjects itself to discipline. For some reason, I suspect that Oak Harbor wouldn't be all that upset if such a situation presented itself.
Others opinions on the subject, of course, may vary; that was just mine.