The way I look at it is: 'Star fleet', as two words, is indeed a generic term that could apply to any space fleet. 'Starfleet', as ONE word, is a proper noun which applies to the fleet operated by the Federation (or the Terran Empire).
To put it another way: The Galactic Commonwealth maintains a star fleet, but not a Starfleet.
those who remember the tranny of the Empire
Admiral Frank N. Furter?