Most of the websites don't even provide this feature.
We are not trying to restrict you from using this feature but we just don't want members to change their name everyday ---> abuse it!!
If we all keep changing names every 2nd day then think, if a member didn't came online for maybe a week and when that member logs in, he will probably feel that he came to a whole new place 'cause he will be confused and we don't want that.
You are allowed to change names, it is not a rule but please do keep in mind that it can 'cause confusion for others.
We didn't come up with this because we were bored or had nothing to do, there is a reason behind it and its for the good.