Here's the simplest answer:
The DIS is family friendly.
Myspace has no real content control to keep people's personal pages family friendly.
So, it was decided that no links to MySpace will be allowed. The webmasters have decided to allow you to say you are so and so on MySpace, much like the ebay decision that you can't link directly to your auction, but you can say in your profile what your screenname over there is.
If you have any further questions about the policy, you can email the webmasters at
admin@wdwinfo.com