Occupied Minecarts don't decelerate on regular rails

To reproduce: Make a long minecart track with powered rails at the beginning, then ride the minecart down the track. The minecarts practically don't slow down at all.

In the screenshot: An empty minecart stopped at the place where the glowstone and minecart are in the picture, while a minecart that the player (or a mob) is riding made it all the way to the end at much the same speed.