By wielding a potent legal threat, the U.S. government is often able to force Internet companies to aid its surveillance demands. The threat? Comply or we'll implant our own eavesdropping devices on your network.

Under federal law, the National Security Agency can serve real-time "electronic surveillance" orders on Internet companies for investigations related to terrorism or national security.

These orders, authorized by the Foreign Intelligence Surveillance Act, are used to feed data into the NSA's PRISM software program that was revealed last month by former intelligence analyst Edward Snowden. PRISM documents indicate that the NSA can receive "real-time notifications" of user log-ins.

Some Internet companies have reluctantly agreed to work with the government to conduct legally authorized surveillance on the theory that negotiations are less objectionable than the alternative -- federal agents showing up unannounced with a court order to install their own surveillance device on a sensitive internal network. Those devices, the companies fear, could disrupt operations, introduce security vulnerabilities, or intercept more than is legally permitted.