Yes, this is discussed in the article I linked to as well. It may be that, although Americans generally have less need to set up shell corporations in Panama, Panama may be relatively more attractive for criminals, since not only can they be anonymous (no different from the U.S.), but also less easily reached through U.S. judicial processes.

This may mean that, though there are fewer Americans named in these files than one might expect, their dealings might be shadier, on average, than other nationalities'.