Some people see gun usage as a sporting activity. Go out and hit some targets, see how fast, or precise you can be, it’s also fun to just blast things. I could easily see a family that shoots together gifting their child an AR pattern rifle after they got used to shooting mom’s or dad’s firearm. It gives them their own platform to customize and practice on, akin to a musical instrument.
That being said, I think it should take a lot more trust, awareness, and scrutiny from the parents, which was clearly missing in this case. This is more like giving the keys for your Dodge Pickup to your teen when they are absolutely hammered.