I'm guessing it's related to their cloud offering. The ability to have a big pool of IP addresses that they can offer to people using their cloud means they don't have to ration them quite as strictly. Amazon already limits you in the amount of elastic IPs that you get out of the box (you can request more http://aws.amazon.com/contact-us/eip_limit_request/ but you need to fill out a form). As things get more scarce, this could become a much bigger issue. Microsoft is hoarding early so that when things really get tight, their users don't have the same pressure.