I agree that ignorance of the law should be part of a defense except for the obvious crimes, because 300,000 laws is more than anybody could know or even understand. This is from Reynolds in USA Today:
"Regulatory crimes" ... are incredibly numerous and a category that is growing quickly. They are the ones likely to trap unwary individuals into being felons without knowing it. That is why Michael Cottone, in a just-published Tennessee Law Review article, suggests that maybe the old presumption that individuals know the law is outdated, unfair and maybe even unconstitutional. "Tellingly," he writes, "no exact count of the number of federal statutes that impose criminal sanctions has ever been given, but estimates from the last 15 years range from 3,600 to approximately 4,500." Meanwhile, according to recent congressional testimony, the number of federal regulations (enacted by administrative agencies under loose authority from Congress) carrying criminal penalties may be as many as 300,000.