No more inhumane than bowhunting which is legal. The whole objective of that is to shoot an animal in the lungs and let it run off and bleed out.

With a bow you have a pretty good chance of hitting your target (the heart) from a long range. This strikes me as ridiculously inaccurate;
this guy is a "pro" and he misses at less than 100 feet, and when he does hit he ends up taking out hitting a random part of its abdomen.

That doesn't strike me as very humane. Keep in mind, I don't really care that much. I eat meat, which means I support the most vile and inhumane practices against animals you can think of. I'm just pointing out that it seems you're going to have to be willing to finish the thing off with a hammer or knife if you want to indulge in this little hunting fashion of the week.

How about hunting with a bowling ball?

I put another video in the hopper of someone who knows how to actually use an atlatl. Yes, they take more practice and the effective range at which a person would be sure of a kill shot would be reduced (based on what I've read a max of 15 yards with an altalt versus the bow's 25), but it's entirely the same general principle as bowhunting.

Needless to point out that a lot of rifle hunters aren't dead-accurate every time they shoot. Lung shots are common. That's why you need a sturdy butt on that gun.

Any time you hunt with the intention of preserving some part of the animal for another use, you run the risk of maiming it or causing it a drawn-out death. Hell, even squirrel and bird hunting often require that you walk up to the dying animal and wring its neck (or beat its head against a tree in the squirrel's case).

I like to think that the weapon is just so awkward that mastering it implies a 50/50 hit ratio when you're aiming for a stationary moose at forty feet.

Good guess. Yeah American Indians and related cultures used them before bows and arrows. We know this mostly from cave drawings of people carrying and using them on buffalo. Another name for the technology is, creatively, "spear thrower." I think this term is used more in Europe and Asian archaeology but it's essentially the same idea.

They're a lot lower velocity than bows, but still faster than chucking it with your arm, due to leverage.

Spears are also heavier, so for the same amount of energy input you get a slower traveling projectile.

besides the time taken to place the spear in the thrower while the rhino charges at you

Yeah, but the bow turned us all into wageslaves of the Catholic church and the federal government.

^