How about filming police, or people activity participating in crime? And if you allow exceptions for that, who's to decide what constitutes crime and what happens to people who film perfectly legal acts that look suspicious?
You can film the police and film public figures all you want, as long as they're acting in their public capacity. You're allowed to film Angela Merkel doing her state visit, going to the beach with Barack Obama and later publish that clip. You're not allowed to film her naked in her private backyard.
The border between the two is sometimes hard to determine, so there are some lawsuits about that, but all in all it's pretty clear.