Because she's not really selling anything. She's giving tutorials and access to her files in exchange for donations. Fan art is a gray area anyway. A lot of artists sell fan art at comic cons. Some even put them together with collections of their work and sell them as print copies of their sketch books. Technically the copyright holders could stop them, but there's so much of it out there they'd spend more money trying to than its worth. And in some cases they wind up hiring the artists to do work for them.