I don't think it helped that leftist organizations in the U.S. such as Weather Underground turned violent. An offshoot of WUO later murdered people to rob a Brinks truck to try to fund further violence.

Using violence to make your point has been a good way to get that point tainted in the minds of most of the U.S., especially after the Atlanta Olympic bombings, 9/11, etc.

Liberal and socialist have also started to take on a negative connotation, but not as bad as 'leftist'.

And of course it bears keeping in mind that many of those who oppose NSA surveilliance on principled grounds also oppose any type of leftist/socialist government by the same principles.