Actually many professional forums do not have an ability to change your username without having an admin do it for you.  Hence paying an administrative fee.

It also curtails the overuse by those who might decide to constantly change their names at a whim and causing confusion.  Particularly in a business environment where names are reputations.

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I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.
-Galileo