People have different preferences for behavior. It's not as simple as doing "whatever you want". Those preferences do not become null simply because one does not believe there is a purpose to existence. While I would like to do whatever I want, there are consequences for my actions, and I would rather not be subject to those consequences. And I would not necessarily say that everything is "meaningless", it depends what you mean by "meaning". For example, the laws of physics cannot be violated, so I would say there is at least some meaning, if by that it is meant some things can be comprehended.