Model sheets CAN be copyrighted:
Model sheets are not typically in the public domain, but are copyrighted material owned by the animation studio which created it. There used to be a lot of model sheets on the internet, however many[who?] studios demanded that they be taken down.
Although model sheets originally are intended for artists who work for the studios that own the characters for which these templates are developed, other artists, such as those who create fan art, profit from them by adapting their characters to their own uses. In addition, parents often download and print model sheets as free coloring books for their children.