Why can't I pooh in public? I mean, its a natural part of life huh?

Mm, because even if it's a natural part of life and is a daily process (or weekly, for the constipated people out there), most of society still deems it an inappropriate behavior that cannot be carried out in public view. Depends on which society and culture you're in though; what may seem totally natural to one may make you seem ridiculously out of place in another.