This is not really true. Left-wing political groups are still actively suppressed by police organizations on a regular basis, as well as Islamic groups (religious and secular), LGBT rights groups, and generally anyone involved in protesting. Police see protest groups as antagonists and act to disrupt them pre-emptively if possible.
The sources for this claim are broad; I would recommend looking into the leaked maryland state police spying documents (which show infiltration of non-violent activist groups), the FBI, local police, and bank conspiracies to disrupt the Occupy movement (long before any actions in the real world had been conducted by any group identifying as Occupy), and the Green Scare.
Combine this with "free speech zones," protest permits, and other before-the-fact restrictions on free assembly, and you can see very clearly how political speech outside the mainstream is suppressed in the United States.
(Interestingly, right-wing groups, white nationalism groups, and similar are not often targets of these investigations and are often protected by police during their protests. In many cases this is the result of local connections, but you have to wonder why animal activists are such a higher priority than white nationalist extremists, especially after Charleston.)