Right, we are more worried about competing experiments seeing our data and discussions. (There may be other concerns, but I'm not a legal expert so I can't comment on that.)
The most compelling reason for confidentiality I've heard is that if another experiment sees our work in progress, it would influence them, and the results would no longer be independent. If you go back and look at old measurements, you find that they were often biased towards rumored values or other experiment's measurements.
(There are also less then convincing reasons, many people fear releasing "unfinished" information to the public would make a bad impression and risk funding. I'm personally inclined to let the public see more how the sausage (err, science) is made. People get the wrong impression that we turn on a machine, get the result, and write it down for eternity into a textbook, but in reality it is always a long an iterative process with many mistakes along the way.)
