I'm guessing once they are ground up they are used for fertilizer.
I imagine that this is the cheapest method. If you killed them some other way they would still be ground up, so it would then become a 2 step process as opposed to the 1 step process they use now.
I am an omnivore and I don't have guilt about eating meat, but senseless slaughter is uncalled for. Humans were designed to eat an omnivorous diet and eating meat is the easiest way to get many essential proteins, amino acids and fats that we require. And the analogy to the bear doesn't work. First off if the bear ate you, then it was probably hungry which then you couldn't fault the bear for. You very rarely hear of animals killing for no good reason. They don't often engage in senseless slaughter. Animals mainly kill for only a few reasons; it's hungry, something invaded its territory, its trapped and must fight, or to ensure it is their genes that are carried on (in the case of social animals when a new alpha male takes over). Of course, there are exceptions to this, but it is rare.