Would be a strange reason. After all, betas and demos are supposed to get people interested enough to buy the game, and great-looking visual do sell games for the casual gamer. For comparison, practically nobody had the hardware to run FarCry or Crysis on maximum settings when they were released, but that did not stop Crytek releasing demos with maximum details allowed.
Also, since betatesting is supposed to reveal problems in the game, shouldn't the max settings be included just to see what happens to the game when a client runs out of resources - does it cause warping, lag for others, what?
Oh well. Maybe they think that if a max-detail beta is released, too many people will get enough of vanilla BF3 before ever buying the retail version, so they hold the full visuals back as an incentive to buy it.