I have never understood the reason why some designers charge a fee to have their copyright removed from their work. No matter what anyone pays, a work always belongs to its artists.
The idea is very simple. I created the object so I own it. You want to use it, then you need to pay me. For example, when people buy custom themes, you need to specify what you get for the price, ie, $100 to use it but copyright removal might cost an extra $50. The reason is simply because you haven't purchased the right to remove the copyright when you initially purchased the item. Therefore, this incurs an extra cost.
Copyright's on themes & plugins are typically BS anyway. I highly doubt developers take the time and money to officially apply for and register a full legal copyright with the Library of Congress & United States Copyright Office.