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For context, here is the law in question. It seems pretty straightforward, until you get to this part:

(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.

So if you are under 18 and you are using a rifle or shotgun, you need to be in violation of 941.28, or you need to be in violation of 29.304 and 29.593 for this law to apply. You could interpret the law another way, but the Judge interpreted it this way.

941.28 has to deal with short-barrel rifles. So the defense argued, and the judge believed, that the prosecution would have needed to prove the rifle was a short-barrel rifle, which they did not.

29.304 and 29.593 deal with restrictions on hunting, and you have to be in violation of both sections. Since Kyle wasn't hunting when he violated the law, clearly this wouldn't apply to him either. Hence the dropped charges. Edit: Other commenters have pointed out that 19.304 deals with restrictions on hunting while under the age of 16. Since Rittenhouse was 17 at the time of the shooting, it's impossible to be in violation of that one.

Gaige Grosskreutz on the other hand admitted carrying a concealed weapon without a license.

TL;DR: It is legal for people under 18 to carry rifles, so long as they aren't a short-barrel rifle, and you aren't violating other hunting laws. But, you can't carry a concealed pistol without a permit.

Why did the Prosecutor choose to charge Kyle with a legal gun, but not his own witness with an illegal one?