This is because Dynadot don't want to put yourself on security threat. Lets have an example:
What information a hacker needs to hack into your account, 1. User Name, 2. Password
If a hacker is able to grab your User name from here, his 50% job is already done.
And if you still try to change the forum name to user name you will receive this error "For security, please don't user your username to post in the forums."
Though you can change the forum name to what ever name you want from here: https://www.dynadot.com/account/info/settings.html
[This post has been edited by mashahood_niz on Feb 13, 2008 7:08am.]