Modding is a strange thing in the world of PC gaming. For years it has existed as a fairly underground thing; something would be game designers would do in their spare time, and something that only the very hardcore would consider doing to their games. When Steam came it seemed even more obscure, until the Steam Workshop took off with its support of a few games, most notably Skyrim.
The Steam Workshop allows players to upload their mods to Steam and distribute them for free. Players can then “subscribe” to mods which automatically download and apply themselves to the game. Valve, the owners of Steam, threatened briefly to start charging for mods, given the popularity of the service. The backlash was astronomical. Such was the force of the pushback from the community that Valve shelved any plans they had and allowed the workshop to continue unaffected. PC gamers are often very vocal and precise in their complaints and, given they were the driving force behind the success in the first place, they were well within their rights.