It's Hit Parader...it does not mean much in my book for these kinds of things. Maybe Metal Maniacs, maybe Terrorizer, but certainly not a publication that has always been in my opinion a rack filler and nothing more.

As much as I like Kurt Cobain's vocals, I wouldn't call him a metal singer either.
Are you sure this is a metal vocalist list?

It's more of a Mainstream-sided definition of Metal and Rock from what I gather.

It was published in Nov. 2006.

only about half the list were metal singers O.o