This is appealing but not actually true. In the absence of copyright, no contract exists for downloading the game client, which means no EULA can be enforced. In the absence of a EULA, there are zero legal barriers[0] to people setting up private servers, which are protocol-compatible with the free game client, and hosting their own game, which destroys the business model of the MMO creator.

This is more-or-less an impossible problem to solve technologically at the moment: MMO servers are (by definition) too contended to do more than act as more than glorified packet switches for authoritative clients. The only lever to pull is to put more functionality on the server and off the client, but for a variety of reasons this leads to a less satisfactory user experience.

[0] http://wow.joystiq.com/2010/06/07/the-lawbringer-the-history... -- a quick history of the action by Blizzard against the maker of an in-game bot, legally identical to running a private server. Note the claims were all either copyright-related, won by the bot-maker, or involved "tortious interference"--i.e. incentivizing the buyer of Glider to break their EULA with Blizzard, which would no longer exist.