I wrote about it a few days ago in response to Troy Hunt's FedEx post
[0]. To answer your question, one of the reasons they don't use their
own subdomains is that it's cheaper to delegate lots of support
services to third party providers. Another is to avoid "polluting"
their domains in the eyes of various reputation filters.
Is it because it's easier to deal with the registrar and pay instead of the internal bureaucracy to assign a domain name?
Is there some technical reason? I know that user-generated content may end up on separate domain for cookie reasons but is there anything else?