[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.]
even if they are native animals, the SIZE of their population is caused by humans, and so they are disruptive to their local ecology. and bell or no bell, letting your cat outside unsupervised is leaving them to get hit by a car, or tortured by mean kids, or scooped up by a predator, or stealth-adopted by someone who cares about the well-being of cats more than you do. I'm assuming you wouldn't be happy with any of those things happening to your cat.