The mods need to be able to have a public face where information about it can be accessed by people who don't own the game. I am not saying people who haven't bought the game should be able to download the mod, merely that people should be able to read about it. That public face is also essential for the mod to get any major media coverage which also happens to benefit Paradox.
Nothing preventing you from creating say a web page for the user mod or a blog or and Ad campaign if you wish, you just cannot have a site with user interaction, so no forum for example, even bug sites should limit the users to people actually working on your mod. example Paradox has a bug site for our games, but we do not let our players directly interact with it. Instead they post issues here and one of us enters it in our system. So as long as you do not post your download link on the web page, we do not have issue with it.
We genuinely like you guys (hey we often even hire from this great talent pool to work with us), and want your mod to succeed, even if you think all corporations are evil, Consider it is even in our interest if you succeed, since people need our game to play your mod. We do have some rules that we need you to follow but I am willing to do what I can to minimize the impact of them on you. truly we want to be your partner in this.