Everyone is presume to know the law?

Nope.

You mean "presumed".

The maxim is often correct, but sometimes not. Why?

Cops and judges will always say "Ignorance of the law is no excuse", right before they give you a ticket, or put you in jail.

Ignorance of the Law Is No Excuse - Reason.com (http://reason.com/archives/2010/08/02/ignorance-of-the-law-is-no-exc)

Ignorantia juris non excusat - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat)

So what's your story?