According to the EFF, “The treaty requires that the U.S. government help enforce other countries’ ‘cybercrime’ laws—even if the act being prosecuted is not illegal in the United States. That means that countries that have laws limiting free speech on the Net could oblige the F.B.I. to uncover the identities of anonymous U.S. critics, or monitor their communications on behalf of foreign governments. American ISPs would be obliged to obey other jurisdictions’ requests to log their users’ behavior without due process, or compensation.”
Read this article from the Washington Post with commentary from Ars Technica.