Um, NOT true in Wisconsin. The signs have the force of law...here's a quote from the FAQ on CCW from the WI DOJ.

"RESTRICTIONS BY BUSINESSES AND PROPERTY OWNERS
Can a business or property owner limit or prohibit the carrying of concealed weapons on its premises?
Yes. The law permits certain owners and occupants of property to prohibit persons from carrying a concealed firearm in or on the property. A person may be subject to a Class B forfeiture if he or she carries a firearm on the property after being notified not to remain on the property or remain with a specific type of firearm. In the latter case, a property owner can prohibit a person form possessing a specific type of firearm on their property or any firearm. Wis. Stat. 943.13(1m)(b).
In addition, property owners generally possess the right to exclude others from their property. While the specific provisions below only reference prohibiting the possession of firearms, property owners may also prohibit or restrict the possession of other weapons on their property. Violations of such restrictions may also constitute a Class B forfeiture under Wis. Stat. 943.13(1m)(b). Therefore, the language quoted below, while only mentioning firearms, applies with equal force to other weapons."