This policy is not the product of unionization. Blaming unions for it makes as much sense as blaming the phases of the moon for it.
Further, "probationary" and "junior" are not the same thing. People who have recently been promoted or changed roles are in this category, even if they've been working for the federal government for decades and decades.
Once you're past your probationary period, you are likely protected by a union. Therefore it is a side-effect of unionization that these junior-est of employees are the easiest pickings.
Agree that the term "junior" here has some nuance to it, though.
So they might be in the union, but not protected by it yet. Again, "new" or "junior" enough that the admin would rather go after them than "regular" employees.
It's generally accepted that a professional civil service is superior to a spoils system. That it's better to have a civil service that promotes people based on merit and retains experience from administration to administration, than a system where most key people are political appointees serving the current administration.
A professional civil service sometimes comes into conflict with the political component of the executive branch. Because of that, civil servants need stronger employment protections than ordinary employees. For example, the administration should not be able to fire a career civil servant for politically motivated reasons and replace them with a loyalist.
But hiring obviously goes wrong from time to time. A probationary period with a lower bar for firing is a compromise that makes hiring mistakes less costly if they are detected early.
