@yeahhmonkey There's a condition in the mods upload that says.
Any archive containing only original game files
I think it's because transmitting original game files could be classed as a direct breach of copyright. It's probably just to protect the site more than anything. Files being modded means you're not effectively transmitting what Rockstar have released, even though you kinda are.
It turns a black and white area into a fuzzy grey area.