I think you're exactly right, but it is more than political/lobbyist/social pressure. Even if the fed knew there was a bubble, announcing so would coordinate everyone's actions and we would have a crash for sure. If they announce that there is no bubble, then they have some time to try to manage the situation (which clearly didn't work in the housing crash).
More generally, public statements are an important policy instrument for the fed. A lot of macroeconomic research in the last 30 odd years has focused on the importance of people's expectations about the future and how traditional monetary and fiscal policies work once you take into account how they affect people's expectations. For example, the government giving everyone $1,000 may not increase spending today if people expect that this government expenditure will lead to higher taxes tomorrow.
So the fed has to take care and think about how what they say and do will affect people's expectations. As another example, right now we're in a situation where it is hard to the fed to lower interest rates because they are already basically at zero. One thing that they could do is say and do things to make people believe that we will have higher inflation in the future. This should spur companies to make investments today rather than hold onto their cash.
This isn't to say that the fed can say anything and people will believe it. What the fed says has to be credible. There is a lot of interesting research that looks into this issue and shows what it means for the kinds of policies the fed can or should adopt. The 2004 nobel was awarded in this area -- the nobel page has a lot of good information: http://www.nobelprize.org/nobel_prizes/economics/laureates/2...