I can easily see a union boss say "Our Oracle employees' wages will be undercut by these unauthorized developers using FLOSS alternatives. This is unacceptable. It is our job as union representatives to insist on the highest standard of professional software, and demand that all unionized software houses use Oracle."
Much of the opportunity in the software industry comes from software developers undercutting other software developers. You & I like it when we're the beneficiaries, and not the dinosaurs being undercut. There's no guarantee (and actually a pretty strong selection incentive against) the innovative developers being the ones with political power.
Oh, wait...
ETA: more seriously, unions can push for or against bad practices, just as management also can. Broad generalizations almost always represent ideological campaigns more than an objective appraisal of the on-the-ground landscape.
I'm quite sure tech unions would end up being heavily pro-open source, with the result of more code being open sourced throughout the industry.
I say this as the TL of a team at Google that fought (and won) to open source our project. It wouldn't have happened were it not for me and a few other engineers pushing for it to happen.