I finally found something on why officers are so willing to issue a Disorderly Conduct or Disturbing the Peace citation:
http://www.legis.state.wi.us/LRB/pubs/wb/00wb11.pdf
Carrying Firearms in Public. Wisconsin law does not specifically prohibit the open carrying of loaded or unloaded firearms in public, but a person doing so may risk being arrested and charged with disorderly conduct, on the grounds that the display threatens the public peace or safety.
Firearms Prohibited in Public Buildings. Except for law enforcement or private security personnel, persons are prohibited from going armed with a firearm in a public building owned or leased by the state or political subdivision [s. 941.235].
WI Statute 941.235 Carrying firearm in public building. (1) Any person who goes armed with a firearm in any building owned or leased by the state or any political subdivision of the state is guilty of a Class A misdemeanor.
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So, there is no law against open carry, but the courts have an opinion on it. Of course, there is a law against concealed carry.