I guess I don't see the problem. The issue here most likely WERE her actions and performance rather than her actually starting the union crap.
I can't possibly imagine that she was going about this in a brand manner. If she was talking crap about Target on TV, well, isn't that a brand counselling? At least? Target has an open door policy, and integrity hotline (and, coming from HR, this is very highly regarded), and the "let us know" form. All of these are great ways to deal with issues you are having with the company.
Target isn't perfect. Everyone makes mistakes. I was fixing tm's paygrades for quite a while after I moved to HR, for example.
That being said, Target -does- treat it's employees well. And Target does a LOT to make life easier and healthier for you and your family. But, fyi, you're probably not going to get 40 hours working as a cashier, operator, service desk tm.....so many positions are usually part-time. And on the lower paygrades.
Target is no different than any other place. You can't just come in as a seasonal tm and expect to be able to support your family. You have to work up. And Target (retail) is certainly not for everyone.
I doubt that team member was the only one who got fired. If a union vote came to their store, that means that the leadership team was doing almost everything wrong.