[alt text: an illustration of a person with a head-empty expression on their face, who is saying, "Not letting your cat outside is CRUEL!" Around the person are various gruesome scenes of different cats in distress. From the top and going clockwise, the scenes include: a cat being carried away by a hawk; a cat that is on fire; a dead cat in the road that has been run over by a car; several dead kittens; a cat that is missing an eye and various patches of fur; a cat that is feasting on a songbird; and a cat that is being carried away by a coyote. The person appears to be completely oblivious to these scenes of distress.]
I've never understood how this is so controversial
People's parents taught them that cats need to be allowed to roam outside to be healthy and happy. Many people don't have the critical thinking skills to change the views that their parents gave them.