Besides the very valid issue of the danger to cats from idiotic human beings -- reckless drivers and creeps who like to torture animals -- there's the problem of the North American songbird massacre.
I live in an area that was once frequented by many songbirds. The law in this town was that cats were only to be let outside at night, (a special ordinance because of a plague of rats on the university campus) and kept inside during the day. Most people in this neighbourhood pay no attention to the law and their cats are outside 24 hours.
I have personally witnessed my neighbours' cats attacking and murdering songbirds in my front yard -- just for pleasure, they don't even eat them! Why should they? They are very well fed at home. They are sport hunters, often working in teams. I've confronted my neighbours with the evidence -- the torn and mangled carcasses of once beautiful songbirds -- and they are in complete denial that THEIR little darlings would ever have done such a thing. Must have been somebody else's cat. Yeah, right.
National Geographic did a study several years ago about the huge reduction in the songbird population in North America. Urban sprawl, pesticides and the domestic cat were specified as the three leading causes of the songbird holocaust.
Christopher Majka's 1999 article 'Cats: Pets or Pests?'" reveals the truth about the devastation caused by uncontrolled pet cats and feral cats. If you think this was just recently discovered in the 2012 study, read this.
The domestic cat is not a natural species. It evolved from small wild cats which were kept by human beings starting many centuries ago as an INDOOR pet to help get rid of vermin. Along with snakes, cats were venerated as creatures whose clean habits promoted long life and good health. Cats kept indoors live longer, happier, healthier lives. It's too easy for them to pick up costly diseases such as worms and feline AIDS if they're left to roam outside.
I have had some amazing, wonderful cats. They are intelligent and loving, if people take time with them. Unfortunately, a lot of times it's a question of some little girl whining for a kitten and then getting bored with it when it's older and not cute any more, so it's let loose to roam free and join the cat equivalent of street gangs.
ABC's Indoor Cats Resources
Why Cats Belong Indoors