PRISM is a national security surveillance program that was launched by the United States National Security Agency (NSA) in 2007. PRISM allows intelligence agencies to monitor the phone, e-mail, and other communications of U.S. citizens for up to a week without having to obtain a warrant. The existence PRISM (also known as US-984XN) was outed by Edward Snowden. Google, Facebook, Microsoft and several other companies denied that they have participated with the NSA voluntarily.
According to The New York Times, Yahoo! tried hard to fight against participating with PRISM because they believed it was a violation of the Fourth Amendment (guards against unreasonable search and seizures). There is a document that goes back to 2008 that shows how an unnamed company (Yahoo!) petitioned the secret Foreign Intelligence Surveillance Court (FISC) to review an order from the government. The FISC believed that Yahoo!’s concerns were “overblown.”
Apparently Yahoo! could not present any evidence of actual harm so they were forced to enter the program. When Yahoo! was outed as one of the participating companies in PRISM, they simply said that the company has “not joined any program in which we volunteer to share user data with the U.S. government.” Since Yahoo! is not “volunteering” in providing the data with the government and is being forced to be a part of it, the statement was accurate.