They taste good, mate. Plus, they'll damage the local ecology and starve themselves to death given free reign, by breeding way beyond capacity and eating everything.

Modern development means that in most places, there's so much surface water available to them that the only limiting factor on their population is food availability.

An adult roo has no predators, as the indigenous people helpfully wiped out all the giant goannas etc. long before whitey got here, and the only thing that was keeping them in check after that was water supply. It's for their own good, our good, and the good of the environment to shoot some kangaroos. They make excellent burgers, and sustainably sourced pet food.