>>347687880
Some japanese loophole that lets people make models of copyrighted material, the limited time sale implies you're only getting paid and compensated for the time you spent making it (like if someone commissioned you to make a personal use model) and not getting continuously paid by exploiting the copyright. 
Basically japanese companies want people making fan mmd models of their characters for use in mmd videos because it's free publicity. But they don't want people making money from their IP by making and selling 3D models. But at the same time they realize these things take a lot of time and effort and the creators probably do need to be compensated in some way.