Free and non-coercive is the key. Things such as Card Check mean that employees as individuals are able to be coerced to sign the card rather than unionization via secret ballot. Furthermore so-called “closed shops” infringe on an individual’s right to not join the union (or pay required representation fees.)
Free and without coercion is fine, but in union activities in the US, it is rarely, if ever the case.
Also look at how “scabs” are treated by union employees. They are often subjected to violence and threats if they choose to exercise their choice to ignore a strike. That is the opposite of free and without coercion.
And no, you aren’t coerced to work for any employer. You freely accept employment conditions in exchange for the agreed upon wage. However when unions get involved, you don’t have a choice — you get paid whatever the union decides. A “software engineer 3” is paid whatever the rate is for a software engineer 3. You can’t negotiate anything better. And, raises are determined by how long you exist at the job and not necessarily your productivity. Traditional unions were designed around workers that were interchangeable — assembly line type work where worker A and worker B are completely interchangeable: as long as the widget screw gets installed at your spot on the line, the work is done: there is no difference between workers doing the same job because it’s rote. When things like judgement, creativity, and other soft skills come into play, workers are quickly differentiated and don’t necessarily deserve the exact same wage because their output isn’t exactly the same as it would be for an assembly line worker.
Join a union if you want— that doesn’t give the union the right to negotiate on my behalf against my will. Unions are no different than labor monopolies and they should be treated with the same suspicion as we treat other monopolies.