If you ask a non-lawyer to identify a principle of law, he or she is likely to respond with "Ignorance of the law is no excuse." For much of our history, this would have been a perfectly sensible response. Traditionally, the purpose of the criminal law was the punishment of those who wrongfully caused harm to others, not the regulation of interpersonal affairs. One does not need notice of what the law requires to know that one should not intentionally harm one’s fellow citizen. The mens rea requirement of the criminal law embodies the fundamental principle that punishment requires personal fault.
As legislative bodies have exhausted every possible permutation of malum in se offenses, the ones everyone knows or should know are wrong without having to be told in explicit detail, they have increasingly crafted malum prohibitum to be used as a regulatory framework to control more behavior that isn't inherently wrong, but that they have decided for whatever reason shouldn't be done. These offenses don't necessarily involve any moral fault on the part of the perpetrator, but rather a choice between various options, one or more of which has been denominated a crime.
The concluding paragraph sums up the position nicely:
By passing statutes that criminalize innocent or merely negligent behavior or that are so broadly defined that citizens cannot be sure when they are violating the law, the federal and state governments have significantly eroded the traditional mens rea requirement for criminal conviction. This is a development to be much regretted. There are many things a liberal government may do to improve social welfare. Government may properly ask individual citizens to make significant sacrifices for the common good. However, there are also many things a liberal government may not do. Visiting the opprobrium and stigma of criminal punishment on those who have not behaved in a blameworthy way is among them. Such official scapegoating is inconsistent with a liberal legal regime. A just legal system does not permit punishment without fault. Hence, justice demands the reinvigoration and preservation of the mens rea requirement for criminal punishment.
Even amongst lawyers, even criminal defense lawyers, this has become an increasingly troubling issue. Criminal law has become result oriented, to the exclusion of any concern about mens rea. When someone is killed, a crime must have been committed. When someone is harmed, it must be because of a crime. The fact that the conduct was innocent or negligent no longer seems to deter the demand for punishment. It's all about the outcome and that every harm must have a crime to combat it.
Even Doug Berman has difficulty finding this to be too much of a problem:
Ironically, I am not as troubled as the author of this letter by the use of the criminal law to achieve certain regulatory ends. What does trouble me greatly, however, is the use of severe criminal punishments in the absence of serious culpability.
Of course, this assumes that once criminal law is used to discourage socially undesirable conduct, the slippery slope tends to encourage increasingly harsh penalties if the effect of the law isn't easily achieved. One need look no farther than the draconian War on Drugs to find an example of this affect.
The notion that innocent or negligent behavior should be criminalized is one of the foundational problems as legislatures continue to increase the number of crimes and the volume of human conduct that subjects people to criminal punishment. Rarely does anyone ask whether there is any moral culpability attached to the conduct at stake, and rarely does anyone care whether the crime sweeps in the innocent along with the guilty. We want to stop a result, and we're prepared to take no prisoners in the effort. But crime isn't what comes as a consequence of conduct, but what motivates the conduct in the first place.
This is indeed a development that compels a return to the basics of criminal law, the mens rea requirement. This is not to say that law cannot or should not be used to encourage or discourage behaviors that impact on society. But it is to say that converting otherwise innocent or merely negligent conduct into a crime is not the way to do so.