I honestly don't know. I don't have direct experience with junior golf, and don't know what other organizations for juniors do. But you know I'm an old and set-in-my-ways guy, I think its at least a little rude to repeatedly walk away while someone is still completing a hole. This policy seems to me to be a measure that AJGA has taken to address previous issues, and so may not reflect what golfers should consider normal in other competitions or situations. If so, its training young players to be kind of rude, at least in my book.
Did you read the post I made where I showed and linked to the AJGA policy of doing just this?
The Hurricane Junior Golf Tour made a group ahead of my daughter's enact this form of play, going so far as to tell the first girl to putt out to tee off on the next hole when she got there.
You can't blame the kid for doing exactly as he's likely been taught to do.
For it to be "rude" it would have to include the intent behind it. I could make the argument that since the kid was taught to do this, he views it as being rude to stay because in his mind that would slow down play. So he's being courteous by moving ahead and preparing to tee off on the next hole.
It depends on the situation as far as I'm concerned.
If it's a foursome and one guy has either holed out or picked up and he goes to sit in the cart, I don't care. If I'm left alone on a green to finish up that would bother me.
Maybe the latent thought there is that nobody gives a damn any more if I hole out or not.