All federal employees that are unionized are free to pay union dues or opt out. The exact same thing as "Right to work" laws. The problem is the union is still required to represent them by law.
You shouldn't be able to have your cake and eat it too. If you "opt out" you should also lose union representation and protections. I despised some of the union's policies, especially when they took political stances outside of actual labor talks and contract negotiations. They represented many individuals that should have been fired but were not.
Dues were just under 1% of straight time pay and I payed it because my economic interests outweighed their collective BS. Very few people actually opted out, maybe 3 or 4%, half of the members were conservatives, many of them career military men and women. Veterans received additional points on the entrance exams as a reward for their military service, hence the high percentage of veterans in the workforce.
The bottom line is always the bottom line. People seek union pay and benefits because most other options are inferior in every way. Virtually all businesses could avoid unions if they paid more, most can't afford to, many would refuse to. Most employees in the USA are blue collar working stiffs, what other options have they?