It’s a little bit confusing, but from what I’ve read, the collective bargaining rights that they previously enjoyed were granted by the agency’s administrator, so it follows that they can be revoked by the agency’s administrator in turn.

Here’s a 2011 NPR article covering when they were initially granted those rights.

As always, this is the danger in allowing such rules to be set by the executive branch instead of codified into law — when the next guy is in office, they can always undo it.