They can choose it by quitting only, as the work changes before and after the union. So instead of having the unhappy people quit when they perceive the company as not being good enough for them, the unhappy people create a union and make the happy people quit instead. It's fine either way by me but it feels a bit pointless.
Not necessarily. Unions can and do negotiate terms that protect union members to ridiculous extents at the cost of making what is basically an underclass out of non-union employees and employees of other unions. You'll find terms that limit certain scopes of work exclusively to employees of a certain union, and even employees of that union with a certain standing within it. Some of this work is gravy train shit that you'll never get to see if you're not in the union or not in the right union. In some unions, working non-union jobs can result in repeating fees for the length of time you do it. Do it long enough and you'll be so far in the hole that you'll effectively never be able to pay it back, you'll never be in good standing, and you'll join the rest of the employees outside of your union that can't draw crazy high pay for very easy work.
Have you ever worked in a union shop, or have family that did? Demarcation is not a hypothetical case, and it's not something that's so uncommon that you can just not worry about it, especially as a worker that needs to consider whether or not your opportunities are going to be hindered by it. Me, my father, my grandfathers, my friends, their fathers and grandfathers, and basically my community on the whole have watched it happen firsthand or have been directly affected by it. Union membership can be a big choice where you're heavily incentivized to join the union not just on the merits of them protecting you as a worker, but because they're going to work against you if you don't. Some unions have worse problems with it than others, and some don't have the problem at all, but it's not something that is so minor that you can just ignore it.