A closer look at the First Amendment question reveals that 48% recalled that the Amendment protects the right to freedom of speech. The other rights protected by the Amendment (freedom of religion, freedom of the press, the right to petition the government, and freedom of assembly) are far less well-known. For example, only 15% remembered the right to freedom of religion.
These survey results do not come as a surprise to experts on political knowledge. They are largely consistent with previous data going back several decades, showing widespread ignorance on a wide range of legal and political matters, including the Constitution. As Cilliza points out, the Annenberg results paint an even bleaker picture than earlier polls. For example, surveys typically find that some 35-40% of Americans can name the three branches of government. The Annenberg figure of 26% is unusually low. But even if it is an aberration, the results from previous surveys are nothing to write home about.