I dunno... Just let him outside?

Probably for its own safety. Cats can easily be prey for coyotes and foxes and can be hit by cars. Also, cats kill billions of birds a year. They are not natural predators, and are not good for local ecosystems.

I hope no one keeps their cat inside just because it may not be good for the ecosystem

Of course this is my feeling, but I doubt many would keep cats in because of ecosystems. Every cat owner I've known personally has kept a cat inside solely for safety reasons.

Or maybe it's a deaf cat. I used to catsit one, and she was only allowed to go out on a leash because she couldn't hear the cars coming. She even got run over as a baby, broke her lower jaw in two, but gladly they could save her.