I would think that we wouldn't want anyone....an employer or a union....to force an individual to be in a union or to not be in a union. I mean, it busts up the union's solidarity of the workers make their own choice to leave the union, but the law doesn't force them to do so, does it?
I get your point that those people may benefit from the current wage and benefit packages...but if the union is weakened and people see that they've lost their leverage to bargain with their employer (so they lose some wages and benefits), they'd be equally free to join the union and pay dues again to strengthen it's ability to bargain for them.
Not trying to start an argument here. I've never been in a union so I don't get the context, but I think it would bother me to not be able to make my own choice on something like this.