SINCE MAY, AS protesters around the country have marched against police brutality and in support of the Black Lives Matter movement, activists have spotted a recurring presence in the skies: mysterious planes and helicopters hovering overhead, apparently conducting surveillance on protesters. A press release from the Justice Department at the end of May revealed that the Drug Enforcement Agency and U.S. Marshals Service were asked by the Justice Department to provide unspecified support to law enforcement during protests. A few days later, a memo obtained by BuzzFeed News offered a...