That does happen and is definitely worth trying to fight. I know of copwatchers that have been charged with interfering, obstruction, obstructing aid to an officer in immediate danger, etc., but have generally been successful in court. The individual departments policy vary. Some have a definition of lunging distnace where you have to be far enough away you can't easily tackle an officer. Others let you get closer. Another test is if they are letting other members of the public pass by and you have this on tape is a good indication that your presence was not obstructing. If you are on public property, but no other members are passing by it is good to at least take one step back if asked then reevaluate the situation.

In general an officer an charge you with anything. Then you can have the charges droppped in court if they have no merit. But if you want any kind of relief for it you have to show it was a wrongful arrest and/or the officer should have known better than to charge you with these charges.