Good for you. Your opposition still means jack.
Fair use covers reviews, criticism, parody, research, news reporting etc. And even then it's not guaranteed to protect you from breaching copyrights. Case in point: Nintendo getting YouTube videos of their games taken down not so long ago.
No. Steam User Agreement clearly requires you to either be a creator of the content you upload or to have a permission to do so from the author.
If you ever make a mod and state in readme you're not bothered - whatever, let it be uploaded all over the place. This is not the case here though.