Why? The Internet encompasses a lot of protocols. Why assume every domain name supports HTTP? It's not fair to call them incompetent idiots when you're the one shooting in the dark. Without an unambiguous URL, you're only guessing, so accept the risk.
The problem is, the different services could be split across hosts, not just ports. It seems SRV records provide the only option here but they're not supported by browsers. That leaves www as the only viable option to move web serving into a different machine.
Most web sites sit behind load balancers and stateful firewalls. They have many, many opportunities to do the port translation to make $name and www.$name work the same way.
The theory you espouse, that www was the only viable option to move web service to a new machine, was true in the 90s when we started doing this. That's how the practice came about. It's 2011. It's not an issue anymore.
http:// equals port 80 and https:// equals port 443, the 2 ports web servers run on. Regardless of the subdomain, if you have http:// or https:// in a URL your getting a webserver. Having to specify www.whatever.com to reach the web server (i.e a website) is redundant, the URI takes care of that. Plus, the vast majority of people on the Internet don't understand protocols or the relevance of why websites start with www. When the concept of URI's and URL's were developed it was to deal with the myriad of protocols available on the early internet, the HTTP protocol was just one of many, but now that the World Wide Web is essentially the face of the Internet to the average user, I feel the tech community should adapt to the current situation and not force people to adopt our, somewhat outdated, standards. Browsers like Chrome just drop the http:// altogether now in the URL bar and the forward facing websites of organizations should drop www. too. I understand the DNS implications but it's trivial to drop the www. at the webserver level.
Of all the protocols on the Internet, HTTP is the most important user-facing one. Having your primary domain not support it, even on the level of blindly returning 301 to all HTTP requests, is tantamount to boarding up all the doors on your shop and hoping visitors guess you want them to come in via a rope dangling from the skylight.
