Again, when it comes to workers, the left is supposed to be the "good guys".
If it's a closed shop, you can't negotiate that T&C, even if the employer is amenable. That's like Polly demanding that if she works there, Molly can't be given more PTO. Polly can ask for MFN (if anyone gets more PTO, I get it too), though even the MFN rules are getting complicated now. Actions such as these run a foul of various legal rules such as interference with contractual expectancy and antitrust that unions have exemptions from.
And particularly if you already work there, you don't get a choice....lose the election, lose your job......if an employer took that position with an action you didn't like (vote for Trump or lose your job) you'd be screaming the red revolution and workers rights.
It's why there are two ways to skin this cat: Right to work...you can't be forced to join a union (particularly for a public employer union) and the trade off I'd be in favor of is make it easier to certify the union (if Fred and John want to form a union in the mail room, more power to them....if they negotiate a better deal more people will want to join the union). The other way is to make it democratic year over year....prior to concluding a new deal every year, have people vote to see if they want to keep the union. My preference if the former, rather than the latter because the latter is just so much more complicated and subject to employer shenanigans.