Especially those that are new, or gain a lot of sudden attention, like the Tea Party did. Many times they move into these groups just to monitor for intel purposes, they're monitored to watch who is funding them, determine who the players are behind the scenes etc. But it's not just law enforcement. As I said, investigative journalist, political competition, private investigators etc, routinely infiltrate these types of organizations.
Of course, many times it's not the local police conducting these operations, but other law enforcement organizations, like the FBI for instance.
Along with protests/political groups, even companies are routinely infiltrated by competitors/law enforcement, private investigators and others with whatever motives.
In the 60’s, the single largest source of CPUSA’s revenue was the money the FBI was paying its informants.
Are government organizations infiltrated too? ;)