Why Is It Cruel to Keep a Goldfish in a Bowl?

Just a random thought while I was running at the gym. I don't keep my goldfish in bowls. They are in a 50 gallon with a Fluval 405 canister filter and a 30 gallon with a AquaClear filter. But I have wondered this for a little while, why does everyone say it is cruel to keep goldfish in a bowl? For hundreds of years they were kept in bowls in China and Japan while they were being developed into the fish they are today without filters and water treatments (example of bowls here: a href="http://www.exoticgoldfish.net/setup.html" rel="nofollow"http://www.exoticgoldfish.net/setup.html/a ). I'm just so curious because whenever someone posts about a goldfish in a bowl it's like people upset over it. Is it because we have modern technology and should be using it? Or is it for other reasons?