I have been in several business proposals where things are capped but few times I could found any reasoning on that.
And it seems that I am not alone, because Seth Godin here explain is point of view on it.
Basically if someone is taking risks with you, why limit the upside shall success came and both parties can profit from it? If the deal is balanced and well structured most likely you don’t need a cap. Besides the cap can harm, reduce motivation and limit growth “out of the box” and that way probably both parties loose.