If the coworker is part of the union and you aren't, then the union has fine its job.
The issue is that I think most of us agree when it gets to the level you describe, perhaps that's not a job worth doing.
It's easy to see how it gets to that point though. Friends in the union might not care who is in the wrong, and selective retelling of the facts can get other union member up in arms about the "raw deal" their fellow member is getting, so it's probably easier for the union to just protect all but the most egregious behavior against non union people. I imagine that's just how the incentives work out.
