The State Department labeled the M.E.K. a terrorist organization for a number of reasons, among other things, they staged terrorist attacks killing U.S. military personnel and civilians. They were also funded by Saddam Hussein.
From the State Department’s website:
The group’s worldwide campaign against the Iranian government uses propaganda and terrorism to achieve its objectives. During the 1970s, the MEK staged terrorist attacks inside Iran and killed several U.S. military personnel and civilians working on defense projects in Tehran. In 1972, the MEK set off bombs in Tehran at the U.S. Information Service office (part of the U.S. Embassy), the Iran-American Society, and the offices of several U.S. companies to protest the visit of President Nixon to Iran. In 1973, the MEK assassinated the deputy chief of the U.S. Military Mission in Tehran and bombed several businesses, including Shell Oil. In 1974, the MEK set off bombs in Tehran at the offices of U.S. companies to protest the visit of then U.S. Secretary of State Kissinger. In 1975, the MEK assassinated two U.S. military officers who were members of the U.S. Military Assistance Advisory Group in Tehran. In 1976, the MEK assassinated two U.S. citizens who were employees of Rockwell International in Tehran. In 1979, the group claimed responsibility for the murder of an American Texaco executive. Though denied by the MEK, analysis based on eyewitness accounts and MEK documents demonstrates that MEK members participated in and supported the 1979 takeover of the U.S. Embassy in Tehran and that the MEK later argued against the early release the American hostages.
Before Operation Iraqi Freedom began in 2003, the MEK received all of its military assistance and most of its financial support from Saddam Hussein. The fall of Saddam Hussein’s regime has led the MEK increasingly to rely on front organizations to solicit contributions from expatriate Iranian communities.
Following the U.S. invasion of Iraq, the group was disarmed and has said they have renounced terrorism. They started lobbying to get their name removed from the list of foreign terrorist organizations. The group managed to achieve this in Europe in 2009, and eventually, the U.S. in September 2012.
By accidentally signing this invoice and somehow failing to notice that it was addressed to their opposition, the M.E.K. outed itself as the ones pulling the strings at the National Association of Iranian Academics in Britain.