Yes, as far as I know the charge damage of a horse determins how well it can knock down people, and at which speed.
I guess the devs want the light horses to be ONLY for transportation means, which means you ride by your opponent, but you are NOT supposed to knock him over with it or damage him. Which is, on one hand, understandable logic. I don't see why someone should be able to not only move through a bunch of enemies, even if they don't see him, but also to knock them down or damage him. And all that would require it was pressing W. If you see it from this point, the bump damage nerf is reasonable.
But if this is the case, I'd like to see a general upkeep reduction for at least light horses. The upkeep used to contain their bumping potential, and as it's gone now the upkeep should be adjusted. About 20-30% less would be resonable, I guess.