And remember folks. Don't try to recreate one of the famous D&D modules or base your adventures on LOTR, use any excisting D&D setting, the Smurfs or any other copyrighted IP.
That is prohibited and most certainly very very illegal!
Do you really see these as the same thing? The difference is that none of those examples you give are going to give a rat's ass if people start making fan fiction (most of them encourage it, though they generally have the legal right to prosecute it). Inkarnate, on the other hand, will mind if you use the software that they expect to be paid for in a manner they don't like. That's why they have a Terms of Use.
It's the difference between someone making a Star Wars mod in Divinity OS2 and someone downloading an illegal copy of Divinity OS2 on the internet. Inkarnate allows you to use their software, subject to their terms of use.