Short Answer: because they can
Long Answer: because the string 'www' does not identify the service provided. The 'name' of a site is just an alias for the IP of the computer serving the site. All transfer is done using the IP address. In order to find out what IP address belongs to which computer, the DNS network is queried, which maps the URL to an address, and one IP can serve more than one URLs. The identification of which service is used is done by the protocol part of the URL, eg http:// or ftp://, which the client application (eg. the browser) will translate to a certain port (eg. Port 80/TCP). Sites like
freshmeat.net even do away with the 'www' part completely.
In larger installations the distinction using the URL is/was done to distinct between server for HTTP (www), FTP (ftp), or E-Mail (smtp/pop).
