What is an illegal act?
An illegal act is any action proscribed by law. In other words, in order for an act to be a crime, and therefore an illegal act, it must be prohibited under the laws of the state (or federal government), and there must be a penalty provided by law if someone is convicted of committing that act. If the law does not prohibit the act, theoretically then it is not a crime.
However, Wisconsin law has a catch-all statute that provides that if some act is not proscribed by law, but it nevertheless wronged another person or society, the state government can still prosecute the person for committing that act. On the other hand, it is unconstitutional for a person to be charged criminally for any act that is not proscribed by law, properly written as law and communicated to the public at large.
Each state is responsible for its criminal code. The legislature can make whatever law it elects.
Helpful information:
Criminal Law Overview
