They don't have to, the law already applies to anyone under 18 unless they're out hunting or a couple other things, none of which applied to Kyle that night.
Again, you're just wrong about this. The law in question says that it only applies to minors armed with a rifle or shotgun if they are also in violation of one of three other sections, as part of that "out hunting or a couple other things". One of those "other things" is carrying a rifle or shotgun that is not short barreled while being 17 years old.
You keep pointing to what is essentially a memo written by some staff of a group of 22 legislators summarizing the law as though it is authoritative above the law itself as written, passed and signed into law. If you don't understand how insane that is, imagine I let 22 GOP Senators write a memo on civil rights law, and then tried to pretend that memo was authoritative over the law as written, would you accept that? No? That's exactly what you are doing here, just at a state level rather than federal.
948.60 has three sections, the first defines a "dangerous weapon", the second makes it a class A misdemeanor for a minor to possess a "dangerous weapon" and a class H or I felony to give them one. The third section lays out the exceptions to this (the ones said memo summarizes as "certain exceptions for hunting, military service, and target practice". Specifically:
(a) This section does not apply to a person under 18 years of age who possesses or is armed with a dangerous weapon when the dangerous weapon is being used in target practice under the supervision of an adult or in a course of instruction in the traditional and proper use of the dangerous weapon under the supervision of an adult. This section does not apply to an adult who transfers a dangerous weapon to a person under 18 years of age for use only in target practice under the adult's supervision or in a course of instruction in the traditional and proper use of the dangerous weapon under the adult's supervision.
(b) This section does not apply to a person under 18 years of age who is a member of the armed forces or national guard and who possesses or is armed with a dangerous weapon in the line of duty. This section does not apply to an adult who is a member of the armed forces or national guard and who transfers a dangerous weapon to a person under 18 years of age in the line of duty.
(c) 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. This section applies only to an adult who transfers a firearm to a person under 18 years of age if the person under 18 years of age is not in compliance with ss. 29.304 and 29.593 or to an adult who is in violation of s. 941.28.
OK, so (a) is specifically about target practice (and a charge against Rittenhouse passes this test), (b) is specifically about military service (and a charge against Rittenhouse passes this test), and (c) is where it gets interesting.
He is charged with having a rifle and is under 18, so (c) applies. Which means that he must be in violation with 941.28 or not in compliance with 29.304 and 29.593 for 948.60 for 948.60 to apply.
29.304 are restrictions based on age (starting with those under 12, and proceeding finally to the 14-16 range). As Rittenhouse was 17, he is necessarily in compliance with it, because none of it applies to him.
29.593 is about getting approval to hunt. As Rittenhouse wasn't hunting, he doesn't need approval to hunt, and therefore is in compliance.
This brings us to 941.28. 941.28 is a prohibition on short barreled rifles and short barreled shotguns. This is why the length of his rifle was a big deal. If it was a short rifle, he would have been in violation, and more or less trivially guilty of 948.60. But since it was over the length, he wasn't in violation of 941.28, and since he had to be in violation of 941.28 for 948.60 to apply, 948.60 doesn't apply. A few staff for 22 legislators summarizing everything I just wrote as "certain exceptions for hunting, military service, and target practice" does not change what the law says.