Because it's a fairly useless rule. The only reasons I can think of that support the existance of this rule are A: To stop the mods from constantly having to change usernames; and B: To stop people from avoiding punishment (repeated or otherwise) through name changes (although this point is fairly unlikely).

Really, why shouldn't people be allowed to change their username more? If somebody wants to change their username for a short amount of time to celebrate something, etc, they're not going to want to have to keep that name for three months.

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