They do beta test it. The problem is, since they're uptight pricks about letting out early info, they disallow regular players from helping out. Unlike every other MMO company on the face of the fucking planet, who actually understand how critical having actual players on test servers is and WANT them to be there, SE would rather shroud everything in their big Jay Pee mystery fog, and put out the fires they cause later on when their pathetically small crew of Atomos testers overlook even the most fundamentally basic game mechanics that is discovered in a few hours after the updates/expansions go live.
They really need to get over this bullshit "mysterious secrets" crap and have normal players go in and test their changes out. Having their player-base beta test their changes for them after they're on live servers is completely fucking stupid, and time and again we've seen how it doesn't work. *DING DING* Get a clue, SE. *DING DING*