An excellent analogy that you might find interesting is located at: www.geektimes.com/michael/culture/humor/items/Privacy-related/TransparentTreesAnalogy.html
American citizens have many encryption options available, as do the terrorists. Despite which fact, the government can decode those messages if they have a warrant and really, really want to find out what is being communicated.
Tortured analogies aside, do you think the government should give up all wiretaps as a tool of law enforcement?