The only game they could tack a sub on is D4, since it is still in development. D1, D2, D2R, D3 have all been released already with no subs. Adding a sub would bring about massive lawsuits.
As D4 goes, it is their game now and their prerogative if they want to add a sub to the game. As a member of a free society, it is your choice whether you pay the sub or not. If you are ok with the sub business, then fine, do it. If you are not ok with the sub business, then do what you do best and quit.