A careful reading of Wisconsin law shows a 17 year old can carry a long rifle. This is the relevant law:

https://docs.legis.wisconsin.gov/statutes/statutes/948/60

People who say it was illegal are going by subsection 2A)

Any person under 18 years of age who possesses or goes armed with a dangerous weapon is guilty of a Class A misdemeanor.

But what they are falling to mention is subsection 3 lists all the exceptions to the law!

This is what 3c says:

This section applies only to a person under 18 years of age who possesses or is armed with a rifle or a shotgun if the person is in violation of s. 941.28 or is not in compliance with ss. 29.304 and 29.593.

Kyle was in possession of a rifle so he meets this exception of if he doesn't violate the other stuff.

941.28 covers short barrel weapons, so not applicable.

29.304 covers under 16 year old minors so not applicable

29.593 covers hunter education, so not applicable

A 17 year old in Wisconsin can legally carry a rifle or shotgun!