They certainly can do as they please, but I think many of the things they do are not particularly good for the consumer, especially in this case. The deal is also pretty atrocious for the modders (you only roughly get 1€ for each 5€ item you sell) and given that the early access model has shown that people like to get incomplete content out, get paid for it, then abandon it to start something new so they can get paid for something else, I'm not even sure if this is good for getting high quality stuff out there. Although I definitely believe people should be paid for some of the amazing things they make, I think the way Patreon does it might be the only way forward in providing people with a financial incentive to mod, but at the same time undermine pump and dump patterns of behavior.