Well, she's not the only civilian to work on a Starfleet vessel (Kes, Neelix, Seven, Janet Wallace) but Star Trek has traditionally demonstrated that the Starfleet crew is Starfleet trained and either enlisted or commissioned. Even Spock and McCoy in TMP were drafted or reactivated for a single mission when they could have been civilians.
I suppose one could argue that we don't know that nameless medical staff are enlisted but Simon Tarses, a lowly med tech, was enlisted.
Its an arbitrary distinction in the official record I guess, being formally part of the crew and being assigned to work with the crew.