That's true for most television directors, although series often have directing producers on staff these days. Bowman held such a position on The X-Files and now has the same function on Castle. Other examples of directors on staff would be Jon Cassar on 24 and Michael Rymer on Battlestar: Galactica. Having said that, you're absolutely right about Bowman. He was a "director for hire" on Star Trek: The Next Generation, and certainly wasn't fired. That rarely happens on episodic television, since the schedule is over so quickly that it's probably easier to just let a troublesome director finish their work and never hire them again than it is to go to the trouble of firing them.