Which the modder has already purchased.
When Bethesda buy a license for a tool like Maya, they pay an up front fee and that's generally the end of it. Sometimes you might get small royalty agreements here and there, but Autodesk are not going to take 75% of every copy of Skyrim sold. Bethesda made the game, they have already been paid for the game, Valve have already taken 30% of that sale. Now they're taking 75% between them of content they either a) did not create at all or b) have already been paid for. It's fine that they take a cut but this isn't a cut, it's 3/4 of the entire revenue stream.