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First off, many creators have created content for free and a LOT of content for free too. A good example is the NYBT team that creates lots for SimCity 4, they have created many many many buildings in effort to bring as much of New York to SimCity 4 as they possibly can and they have done so for a very long time. They have done so for free for all this time.

Now, had you come with something like an optional donation system, then I would be on board with that. But if you want me to pay for content you create... Well, you will never get a penny from me. Jus' sayin'.
 
The topic seems to have two arguments. There's an argument for creating content for the game that is payware, which is totally different than trying to make 3D models and hosting it as payware on another website for general purpose, which is completely legal.

The former could be illegal. It may not be, but in my opinion, I'm leaning strongly towards it. Games like these don't charge people for their content, specifically because of what's written in the EULA, and, most importantly, because of the modding. If someone paid for the content and then hosted it on Steam Workshop for others to enjoy (like it or not, there's nothing saying you can't), then half of the map would be blank, or corrupted. The same thing goes with making payware content for this game. If I remember correctly, .FBX is pretty flexible so it wouldn't be hard for someone to backtrack, place it into a 3D modeling program and re-upload it to another website (or the Steam Workshop, in all the audacity) for free.

I've learned over the years that in order to protect your work, you have to have the mindset of the person who would try to steal it. Sometimes, doing it for free beats the hassle of trying to make it payware.