Oddly, the Iowa Supreme Court doesn't find it necessary to consider the laws of other states when ruling on a case, and the states that have these laws for some reason don't find Iowa Supreme Court rulings to be binding precedent.
This was a US Supreme Court ruling, not Iowa.
Katko v Briney is a 1971 case.
The Iowa Supreme Court looked to jury instructions 2, 5, and 6. They were:
“In instruction 2 the court referred to the early case history of the use of spring guns and stated under the law their use was prohibited except to prevent the [**7] commission of felonies of violence and where human life is in danger. The instruction included a statement breaking and entering is not a felony of violence.
Instruction 5 stated: “You are hereby instructed that one may use reasonable force in the protection of his property, but such right is subject to the qualification that one may not use such means of force as will take human life or inflict great bodily injury. Such is the rule even though the injured party is a trespasser and is in violation of the law himself.”
Instruction 6 stated: “An owner of premises is prohibited from willfully or intentionally injuring a trespasser by means of force that either takes life or inflicts great bodily injury; and therefore a person owning a premise is prohibited from setting out ‘spring guns’ and like dangerous devices which will likely take life or inflict great bodily injury, for the purpose of harming trespassers. The fact that the trespasser may be acting in violation of the law does not change the rule. The only time when such conduct of setting a ‘spring gun’ or a like dangerous device is justified would be when the trespasser was committing a felony of violence or a felony punishable [**8] by death, or where the trespasser was endangering human life by his act.”
The Court further stated, [*662] The legal principles stated by the trial court in instructions 2, 5 and 6 are well established and supported by the authorities cited and quoted supra. There is no merit in defendants’ objections and exceptions thereto. Defendants’ various [**15] motions based on the same reasons stated in exceptions to instructions were properly overruled.