There are at least two kinds of speech restriction - not being allowed to say something, and being compelled to say something. To analogize, it's one thing to be told "you're not allowed to rant about the CEO on social media". It's another thing entirely to be told "you have to make 3 posts a day about how great the CEO is".
These federal employees were not just restricted from publicly criticizing the administration - which is fairly typical for federal employees - but they also had their out of office messages changed without consent to point partisan blame for the current shutdown. That's essentially compelled speech, especially since Out Of Office messages still include the employee's name as the From line.
This is perfectly legal and not a violation of any rights. Companies literally have entire departments dedicated to promotion of their products. If you say "I hate this product and don't want to work on promoting it" you will just be fired for refusing to do your job.