True enough. Possibly Riker only temporarily slowed down to impulse to get his bearings, and then continued towards Earth at warp? But it would be extremely slow going for warp, twenty minutes from Jupiter'ish to Earth... Still a good reason to dispatch a shuttle rather than a starship, such as in "Samaritan Snare", "Neutral Zone", "Mind's Eye", or just about any episode where the heroes are in a shuttle, arriving from a conference or whatnot, and at impulse long before rendezvousing with the mothership. And if the difference between impulse and warp is so small (warp is only twice as fast in this model), perhaps impulse is a smart choice for the shuttle flight?