It’s rather simple: http://www.mspaintadventures.com/salesfaq.html
That’s why. I know it’s old, but let me make it clear to you that THIS is why I dislike homestuck (besides the artstyle).
I understand wanting to protect your IP but this is taking things too far. I hope you enjoy knowing that as a fanartist who sometimes does homestuck related commissions, that you are doing something that is disapproved by them and can get you (in theory) sued. I mean, 2D is just about allowed but 3D art isn’t? Fuck right off!
I’ve had to deal with a lot of copyright blahblah in Game Design about IP (really difficult to protect your ideas there) but I know that if I ever had a massively popular IP and people did a lot of fanwork, I wouldn’t pull this sort of shit on the fans. It’s the fanworks which made this series popular and seeing this sort of copyright shit makes me rage.