Both polar bear and moose along with innumerable caribou are taken every year with .223s, .22-250s, and .243s. That doesn't make it a good idea as repeatable terminal performance with light weight high velocity bullets has historically been difficult to quantify. Still subsistence hunters tend to be very good at what they do, hunting conditions one encounters in the North are different from hunting conditions in the South, and the ethics of the subsistence hunter are different from those of the sport hunter. The typical sport hunter doesn't have a much firearm knowledge or ability and therefore the wisdom in the choice of a small bore is questionable.
Enter mono-metal bullets. The TSXs and similar products from a number of manufacturers produce repeatable results from high velocity small bore rifles, and chances are pretty good you could successfully make one shot kills on quartering away big game with them. But these bullets are not commonly available in factory loaded ammo, and the average hunter is likely to avoid the sticker shock associated with factory ammo loaded with premium big game bullets. The novice hunter is often cash strapped and his hunts tend to be bare bones affairs, so he is looking for the least expensive ammunition available.
Unless the novice has been brought up in a hunting culture where guns, shooting, and hunting are an integral part of his upbringing, he won't know why bullet "A" is a better choice than bullet "B". Then again a 12 or 14 year old kid brought up in a family of hard core hunters and shooters probably isn't a novice. TB's kid is a case in point in that he has access to the best equipment, expert guidance, and he undoubtedly has some previous shooting experience, to the extent that the moniker of novice barely applies. If every novice were blessed with this combination, the chances that their initial exposure to big game hunting would be a positive experience increases exponentially.
By contrast, some novices have to figure it all out for themselves, not all of them are kids, and sad to say, some have never fired a centerfire rifle prior to their first hunting experience. Arming these guys with small bores is not a recipe for success any more than arming them with .300 magnums would be. For these reasons I don't endorse the use of small bore rifles for game over 100 pounds, despite the success claimed by many.