Why can't some people differentiate between principles and beliefs?

They interchange depending on what is at hand or what we envision ahead! Principles are a set of rules that we hold on to regardless of what is before us, at least that's what principled people do. If a principle changes then it isn't a principle it is a belief!

Principles "code of conduct" deal with the inner self, belief "rules of engagement" is more about how one views the world around them. When the world changes principles remain the same! Given that we are influenced by our surroundings it means our set principles are an antidote to beliefs widely held in order to survive/develop an identity! Most times principles develop at a later stage in a life of an individual!

Beliefs widely held change hence following them can be detrimental to a human being with populistic ideas! A human being without his own rules can not be called an individual! An individual isn't a rebel he is someone who knows what is good from what is right and what is bad from what is wrong! Principles filters out the good and the bad from the ugly!

The Parable of the Weeds