I think we're operating on Dampe-doesn't-know-anything physics...
It's the laws of motion at work here. If there's nothing accelerating or slowing you down (or the accelerations compensate one another), you retain your velocity. This is the Newtonian interpretation, considerably more correct than Aristotle's, who believed that objects tend to slow down unless acted upon (he didn't factor in friction, air resistance, gravity and things like that).
In an atmosphere, your top speed is determined by air resistance. In outer space, where there is no atmospheric soup to fly through, so the only thing slowing you down is the gravitational pull of various objects, which with appropriate maneuvering and acceleration can be overcome (utilizing orbits, usually). Hence why for spacecraft an important measurement is delta-v - how much of a change in velocity can it make with its thrusters, and top speed isn't really relevant to anything.
However, considering the space fighters are called aircraft and the officers are called airmen (rather than, say, spacecraft and spacemen or, heh, starcraft and starmen), we might be a bit too deeply entrenched in
space being air to backpedal much on the design (which would include probably removing wings from our spaceplanes if they're not meant to land on a planet) without it becoming unrecognizable). It's also unfortunate that both dogfights and starfighters aren't exactly practical in actual space, either, so I might as well not bother pushing more hardness into the setting, even if I was really looking forward to exploiting my minimal, if probably sufficient physics knowledge and exploring decidedly non-air combat maneuvers.