Because you put less power in it with holding it with just one hand rather than two although on horse back you are likely to make up for a it a bit with speed bonuses.
Well, if you use a spear with one hand, it's going to be a weaker thrust. But if your using both hands, while standing on the ground it's going to be a lot more powerfull.
... Of course it matters when you're on horseback, geniuses, I meant it doesn't matter if you have a shield or not when you're on horseback, because your character only uses one arm. I wouldn't imagine there being a penalty on horseback, OR the penalty is always there on horseback. However, on the ground it definately matters.
Animaton issue me says. You only use one hand with two handed weapons on horseback, right? And I say it'd be much easier to thrust a weapon without a shield - even if I used it with one hand - than when wielding a shield. Try this with a rake.