This is really where eSports will come into its own, in these (I prefer to call them MOBAs) games. Just like in professional sports, the players here will need a union.
The players will only need unions if the companies insist on these sort of anti-competitive contracts. I don't think that a company like Valve would insist on such a clause for competitive Dota, which might make it more attractive to players with the skillsets that make them good at ASSFAGGOTS titles. I've played League, HoN, Dota, Dota 2, and Smite, and I can confirm that, while differences in styles of play exist, many of the core mechanics are similar. This is especially true at high levels, where the most important factors in a teams victory is communication and predictive thought.
Many people in the DotA scene have been calling for players unions, as many of the teams' contracts are not well thought-out and leave room for a lot of exploitation of players (citation: /r/DotA2 has like one posted every other week).
This is the flipside of Riot's agreement: Riot can be reasonably expected to hold up their end of the deal. The details on how pro-esports teams are supposed to make a living and pay their bills are still up in the air, but generally teams that get "sponsored" end up getting raw deals.
But Twitch revenue can help offset the lack of funds. Ex-Brood War player AngryTestie was able to live comfortably streaming whatever game he wanted, from HoN to a bit of SC2 to Dota.
