Because it was the dot-com boom, and what was better than one dot-com? TWO!
But yeah, they then proceeded to put everything under .com.com, so they had news.com.com, cnet.com.com etc. It was painfully stupid.
I believe the justification was so they could use unified cookies across all their properties -- they set the cookie for "com.com", and then it was available to news.com.com, search.com.com, downloads.com.com, etc., in the same way that yahoo.com cookies are available to news.yahoo.com, sports.yahoo.com, etc.
no, they had com.com (e.g. http://news.com.com/). I believe that domain is now for sale. Probably gets a TON of typo traffic.