Riot Games prohibits professional League of Legends players from streaming rival games, claiming it is trying to "ensure a true professional setting" to legitimise eSports.
In an section of a contracts published by OnGamers, Riot detailed games which League Championship Series (LCS) players were not to advertise "during or adjacent to League of Legend content during the term".
The list included genre competitors such as Valve's DotA 2, Smite, Heroes of Newerth and Infinite Crisis. A range of Blizzard titles, including the Diablo 3, StarCraft and World of Warcraft franchises, as well as new card battle game Hearthstone, were also prohibited.
Riot's director of eSports, Whalen Rozelle, has since released a statement explaining that restrictions are a result of the studio's desire to have the game become a legitimate sport.
"We say this all the time: we want League of Legends to be a legitimate sport," he said.
"There are some cool things that come from that (salaried professional athletes, legitimate revenue streams, visas, Staples Center), but there's also a lot of structural work that needs to be done to ensure a true professional setting.
"We recognize there may be some differences of opinion in the perception of pro players' streams. In the past, pro gamers only had to worry about their personal brands when streaming and, at most, may have had to worry about not using the wrong brand of keyboard to keep their sponsor happy. Now, however, these guys are professionals contracted to a professional sports league. When they're streaming to 50,000 fans, they're also representing the sport itself."
Rozelle continued on to liken the restriction to NFL: "Similarly, you probably wouldn't see an NFL player promoting Arena Football or a Nike-sponsored player wearing Reebok on camera," he explained.
"Pro players are free to play whatever games they want - we're simply asking them to keep in mind that, on-stream, they're the face of competitive League of Legends.
"I can't stress enough how these guys in the LCS are on the road to being real, legitimate athletes. This is new territory for a lot of teams (especially in e-sports), because the transition goes from being a group of talented individuals to being real icons of a sport and a league."
Fans have been quick to point out that, in the past, the NFL has embraced other leagues, and even allowed players like to compete in multiple sports at the same time.
In October, Riot Games president Marc Merrill confirmed his Twitter account was hacked, and explained that images of a League of Legends spin-off was merely a prototype.