There is a big difference between copying a scene shot
and making a true doujin cel.

Although Doujinshi is based on a normally copyrighted
character the artwork is original.

If a popular online cel shop tried to sell douji cels
based on other cels they would get in trouble.

If you don't believe me flight back to Japan and open

a web site and start selling doujin cels copied from
other cels or scene shots and a studio will step in.

Also I think the studios did try and sue the some
doujishi a long time ago and lost. But, hey they did
try.