I have seen charges into weaker units were the full affect of my charge wasn't needed, but having overkill is ridiculous when there are plenty of other targets. I would think cavalry would only charge if it had a valid target to charge into, and not just 3 guys. The less luck in a game the better, and this is the worst kind of luck.
Think upon a battlefield in real life. Would cavalry charge across their own lines to be able to attack a "worthy" unit. Would they spend the time to retire to the back lines, move around their own army to relocate in a charge position, when doing so may well take so long that the tactical situation has changed? Better to charge that little unit, route them and knock them completely out to the battle as soon as possible and then turn you attention to other targets. Otherwise perhaps cavalry can just sit out the entire battle waiting for a decent target to show up within the part of the line they can reasonably target.
