[brandonsavage]

The concept that ignorance of the law is not an excuse is certainly critical here. The principle exists since it would be easy to simply claim ignorance.

At the same time, the concept of "mens rea" or "a guilty mind" plays an important part in criminal law. If you knew what you were doing was wrong, and you did it anyway, that establishes intent. For example, you can't typically be punished for trespassing until you've been told you are trespassing (hence signs that say "Posted - No Tresspassing"). Once you continue onto the property, you've committed the act overtly, and are therefore guilty of a crime.