The problem with indoor/outdoor cats is that they eat all they need at home, but still hunt and kill birds. A lot of them. Some species may be risking extinction. Plus, if you live in a condo 13 stories up, that's not possible.
> he would happily peel your face off for putting a collar on him let alone a harness
Takes time and effort, but cats are trainable. I grew up with a murder-mitted beast who didn't put up with anything. My cats now, I take the time to train them rather than let them be violent.
How is this a problem with indoor/outdoor cats? Are you worried about the cat's immortal soul?
Or is it more that you just think cats are a problem in general, but you'd prefer to make the claim more specific than that here?
It's answered in the same post you just quoted. They hunt so many birds they are destructive to the environment and are driving some to extinction.
The massive numbers of them are the problem. The only other pet as common is a dog and they don't hunt birds just for the sake of hunting like many cats do.
The only part of the complaint specific to indoor/outdoor cats is that they hunt even though a human will feed them. That's a complaint about the cats' morality, not a complaint about their effects.
It also assumes without justification that humans don't feed their outdoor cats.
The problem is any cats that wander around killing multiple birds. So many people have cats that even a small percentage of them doing this is super destructive to the local birds.
"Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single greatest source of anthropogenic mortality for US birds and mammals. Scientifically sound conservation and policy intervention is needed to reduce this impact."