I think it can be explained (non-canonly of course) as an active warp field creates a subspace drag, which would also explain why impulse fuel is expended constantly while at impulse, instead of simply reaching the desired speed and shutting the engines off. Of course, it can also be put down to the early writers had no clue what they were talking about when it comes to space travel and later writers had no choice but to continue the trend.
Inertia, was used correctly in The Battle, Picard tells Riker to turn off the Tractor beam on the Stargazer, and let her inertia carry her along, which would seem to support the active warp field theory.