Communism was the absolute defining political issue of the 20th century in America. Today communism seems like a failed political ideology, a cute thing that hippies like along with pot and drum circles. But in the 50s and 60s communism was the most evil thing imaginable, like aids spreading terrorists who block cell phone signals. Hoover in particular had spent his entire life fighting communism and saw it as the greatest threat our country had ever faced. Many of our parents or grandparents had nuclear bomb drills in elementary school, ducking and covering under their desks to protect themselves from the communist threat. Growing up like that, you can imagine the fear and rabid hatred that people had for the ideology.
This isn't a story of "Evil US government subverts wiretapping power to go after peaceful protestor". This is a case of the government executing the will of the people at the time. Hindsight is 20/20 and looking back their actions seem evil, but in historical context the FBI was doing exactly what they were supposed to be doing: fighting communism with everything they had. It may sound silly to us, but back then that was the only fight that mattered. The FBI and the public didn't see it as wiretapping a private citizen, but as a part of a decades long political struggle.
Recently on HN people have been acting like the government can dominate the world through wiretapping. As if the NSA can find out a few secrets and people will bow their heads and be silent. Government wiretapping found a true dirty secret involving MLK and threatened to reveal it if he didn't change what he was saying. Did he stop? No. Did he slow down for a single second? Fuck no. True political reformers aren't cowed by blackmail and intimidation. As long as we have leaders like Martin Luther King, democracy will live forever.