Being able to quit a job over a moral objection is a painted as a luxury to most employees (whether it actually is a luxury or not... I think is less relevant than all the media we're fed reinforcing the fact that having a job is a privilege we're lucky to be allowed)
Thus to depict union membership as protecting workers' ability to avoid taking part in the endorsement of views is nonsensical. It is strictly harder to quit a union than to quit one's job. Because the latter not only involves quitting one's job, but also finding new employment in a different field.
(Did you mean "former"?)
I think you're being more than a little hyperbolic. Unions can't force employers to not hire you if you're not a member (in fact most employers probably would prefer non-union employees because the employees have less negotiating strength).
Not to mention that historically unions have been the catalyst for workers rights legislation (like the 5-day work week, and 9-5 hours, overtime pay, and so on). So even if you aren't a member there are obvious benefits to employees who aren't members of a union (in the long run).
This is not the case. At least, it is allowed in States that have not passed legislation to that effect. Many (usually liberal) States have not done so, as it would weaken the power of unions.
Furthermore, there isn't anything that's preventing those employees from voicing their opinions without a union. The story linked in the OP is exactly that. Unless tech companies are firing thier employers for their political views, employees can make their views known regardless of union status. While we have seen a few instances of that, most were views that I doubt the average pro software developer union person wants to see defended (then again a lot of people proposing unionizing developers want to limit H1Bs so you never know).
Contrast this with people who are against what the union is doing. The only way to avoid this is to leave the union. In some states (including California) this may mean leaving the entire field of work. Right to work states guarantee the ability to work without union membership, but many (most?) tech jobs are not in those States.