Further to BigThumb's post about plants locating in the Southern US. Many of those states are 'right to work' states, which means you can be fired with no notice, for no reason, with no severance pay. And in many of those states, you don't have to be paid vacation pay.
Now, I'm not a member of a union, and never have been, but I'm pretty sure that without unions, we wouldn't have ended up in Canada with 4% minimum vacation pay, and a few odd perks like that. Furthermore, when I was a student, working in a (non unionized) mine, our boss mentioned that, in many ways, it's easier to have your hourly employees unionized, because then all of the rules are itemized, they're all laid out, everyone is treated the same, and no one expects special treatment.
I'm not saying unions are always good, but they sure aren't always bad.