While being ridden, horses have an "idle speed" while in a minecart. Instead of having no speed, it will continue moving, albeit slowly. I believe the game understands this idle speed to be 0, as the horse minecart will completely move through any number of unpowered rails, unaffected by them not being powered.
Comments 3
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.
confirmed for 14w32b