Police officers illegally spying on activist groups?

I just looked in the Washington Post and found an article that was saying that police officers have been illegally spying on activist groups within my state (Maryland). I found it surprising that the Washington Post would put an article like that in the newspaper. What would make police officers want to spy on activists groups that are legally using their right to free speech? Do they feel that they are somehow above the law?

Nothing new. I have been to many protests and events, and there are always officers there, taking down names of anyone who will give it them. People that aren't doing anything wrong, and a cop walks straight up to them and says "give me your name" or "let me see your I.D.". this is obviously an attempt to gather the names of as many activists as possible, surely to enter into some database.

The part that really fires me up about the whole thing is the illusion that they are there to keep everyone safe.