There may "only" be 33 kids that have been killed by the drains but many more have been permanently injured (usually after evisceration). Further, it is thought that many more deaths are probably attributable to pool drains but are listed as drownings since that is the actual cause of death.
Examples:
Here.
Here.
http://www.cdc.gov/mmwr/preview/mmwrhtml/00016693.htmAnd here.
I'm amused by the idea that somehow these kids aren't "supervised". Only someone without kids would make a blanket statement like that. The pool we go to is very well supervised. I also supervise my 8yo. Nonetheless, she is often underwater...playing games, swimming, exploring, etc.. She is an excellent swimmer. I don't get up and follow her every time her head dips underwater. I don't expect there to be a freak hidden danger like a drain that would hold her down so firmly it would suck her intestines out.
Maybe in some of your minds this makes me a bad parent. Maybe you think I don't adequately supervise my daughter. But I also think that if there wasn't a "hidden" danger, and you saw me hovering* to that extent, you would think I was a total nutjob.
*Personally, I think this kind of extreme supervision at 8 would go beyond hovering and border on some kind of pervasive psychological condition.