I'd agree with you, to a point, that Catholicism is a sect of Christianity. However, it seems to me, that when people mean to attribute something to Catholicism, they instead attribute it to Christianity. Most of the Christians I know, don't actually view Catholics as Christians. Most actually believe the Catholic church is the "whore of Babylon" spoken of in the bible. So as you might imagine, they don't care to have the misdeeds of Catholics laid at their feet.
GZ