Didn't Stephenie explain this? With Edward's, Aro's and Jane's pains, it affects the mind (Hearing thoughts and making you believe you're in pain) and Bella has a block against that.
But, Jasper's and Alice powers don't affect her mind. They effect Bella's body or a decision once it has been made and the outcome of it which, again, is outside Bella's head. It's in the real world.
I think I might be wrong, but I'm sure I read it or heard it somewhere. If I'm wrong, could you let me know?