Well, it's actually not.

I'm a happy Steam user, but it has quirks. Steam has issues with some games and their mods. Medieval Total War 2: Kingdoms, for instance. Creative Assembly put out a realism mod that I was dying to try. It was linked through the game-launcher itself. Updating the game to include the mod, though, changed the .exe and invalidated the install. And it's not unheard of for other mods to change the .exe.

Steam is getting better with issues like that, but they still exist.

Read the post I quoted again:
It's bullshit that Steam can or will ban people for attempting to mod their game. I never said that Steam didn't have issues supporting mods for some of their games.