How do you justify taking away my right to choose where and how I work?
Be part of the group or don't work here. My justification is that it drastically increases the strength of the union.
While I may be free to quit a union job, it feels quite gangster to say that I have to participate in one to take a job. I don't think the tradeoffs -- benefits from a union versus costs involved, especially when it's exclusively to pursue a career at a specific company -- favor unions.
That said, I do think unions could be stronger, and get a bad rap. To me, this seems like a problem the larger/stronger unions caused (and still cause - see police unions).
Your language - "be part of the group or don't work here" - is also a major disincentive. Artificially created black and white scenarios are reductionist.
If they are going to screw me over, then I am going to screw them over first.