"The rationale of the doctrine is that if ignorance were an excuse, a person charged with criminal offenses or a subject of a civil lawsuit would merely claim that he or she is unaware of the law in question to avoid liability, even though the person really does know what the law in question is. Thus, the law imputes knowledge of all laws to all persons within the jurisdiction no matter how transiently."
Comments
http://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat
"The rationale of the doctrine is that if ignorance were an excuse, a person charged with criminal offenses or a subject of a civil lawsuit would merely claim that he or she is unaware of the law in question to avoid liability, even though the person really does know what the law in question is. Thus, the law imputes knowledge of all laws to all persons within the jurisdiction no matter how transiently."