Procuring an abortion used to be a felony. Even though the act is no longer criminal, if you did it when it was illegal and were convicted, you would be a felon. Cheating on your income tax or filling out a host of government forms incorrectly can be a felony. Many of these laws do not require intent. The fact you were unaware you were violating the law is not a defense.
Some people violate the law by accident or stupidity, other people are truly evil and vile.
Putting people in jail without regard to the circumstances of their offense is not just. Our laws must be able to distinguish among these groups. As our firearms laws become more complex, more people find themselves in violation of those laws unintentionally. We must keep law enforcement local and we must allow the courts some flexibility. With the exception of some computer crimes, just about any anti-social activity that should be criminal, is already criminal.