You're not grasping one thing. The files are sold through the deluxe version. If you just have the original version, you don't physically have the files. In order to get those files, you'd have to download them from someone who has them. That is what is illegal here. Once you get the files, you are allowed to use them in your mod, but the mod would only work for those who have the files (you can't upload those files for other people, only refer to them in your system). That's the distinction there and why most modding with Civ5 will be perfectly fine.
It's the same way it worked with PTW and C3C, it's the same way it worked with Warlords and BTS.