And, maybe they didn't want to treat the injuries - the animal was going to lose its eye. Sometimes, people realize that with finite resources, the decision to euthanize may very well be a solid financial decision. If the vet bill to remove the eye and treat the animal would have climbed into the thousands of dollars - then it may have been financially wiser to simply replace the wallabee.
For what it's worth, I'm just playing devil's advocate here. But some of the reporting - rabbits killed by smashing their heads against a wall? Sounds horrifying; I'd bet the death was near instantaneous. So, while a bleeding heart jury might find that so displeasing they would hold it against the person putting down a rabbit in that manner, it remains as probably an incredibly effective method of quickly killing the rabbit. Though, I'm aware that some would simply pet the cute cuddly rabbit until it's calm, then snap its head back quickly, breaking the neck. Or, put a broomstick over it's neck, and simultaneously yank the hind legs upward while pushing down on the broomstick with your feet, pulling on the hind legs until you feel the vertebrae in the neck separate - this is considered near instantaneous (and humane.) Or hanging the rabbit by the hind legs and bashing the base of its skull with a 1/2 pipe. Also considered humane. Or carefully slitting both of its jugular veins with a very sharp knife - it won't struggle; it'll continue to be at ease until less than a minute later, its blood pressure bottoms out & it's nighty night. Again - considered humane. These last three methods sound barbaric - like the animal would suffer. It doesn't. And, if you raised rabbits - occasionally, particularly on first litters, the mother abandons the babies. Drowning them in warm water is sometimes prescribed as a humane method for putting them down. Sure, there are a few "omg, the cute bunny wabbits! I'll spend the next several weeks working with an eye dropper around the clock to keep the things fed." But for most people raising these animals, for something like a rabbit - it's just not worth it.
http://www.raising-rabbits.com/killing-rabbits.html
Drowning isn't the most pleasant way to go, and there are forms of euthanasia that are more effective. But, it's far from the more unpleasant ways to go. Many drowning victims (who were resuscitated) report that other than the initial panic, it wasn't really painful.