How not? The heart doesn't decide anything, it has no will to. The brain is the center of the human and does all the work such as thinking, seeing, feeling, etc etc. Ever heard the saying: It's all in your head?
I never quite get the heart thing. In my opinon I think Xemnas brainwashed the Organization into believing they had no hearts for his own meaning. It'd make sense cause Kingdom Hearts grants so much power he'd want it for himself but couldn't do it along so he bread others like him into believing they were getting back their hearts by getting Kingdom Hearts.
Seriously, if they can think I am sure they have a brain and if they have a brain then they can feel because realistically the heart only pumps blood, the brain is what allows people to feel emotion and other wise.
