The concept of "Ignorance of the law is no excuse" (Ignorantia juris non excusat) was really only ever meant in the context of natural laws, e.g. murder, theft, rape, etc. These things are so obviously horrible that anyone should know they are wrong. Hence "crossed over that line knowing what he or she was doing."
In the realm of political laws, ignorance absolutely excuses. Why should it be obvious that importing orchids requires certain paperwork?
.. and why should the penalty be imprisonment instead of "hey, we know you probably mean well, here is what it takes to do the right thing: x, y, z."
I think we need a line between criminality and what i'll call "regulation" -- which, if intentionally ignored should become criminal, but otherwise there services should help people be law-abiding.
In the realm of political laws, ignorance absolutely excuses. Why should it be obvious that importing orchids requires certain paperwork?