The region-locking is due to the confusing state of Scrabble's international copyright. In the majority of the world, toy manufacturer Mattel owns the rights to the game, but in North America, that honor goes to Hasbro. Everything would be honky dory with the Facebook version, except that EA already has an agreement with Hasbro to distribute digital versions of their titles, which the RealNetworks-developed Facebook app would be in breach of if Scrabble were playable in North America.
Fortunately, North Americans apparently aren't missing much: the game is currently suffering from the "beta blues", with its woefully-small user base already up in arms on the discussion boards over some usability issues, as well as the system's penchant for deleting in-progress games. Nothing beats the real thing, right?
[Via Purple Pawn]