I think the general idea is that if you could (legally) go stand in that public space (sidewalks, roads, parks) and watch something happen then you're allowed to record what you see.
This is probably good - I think it's the basis of being able to record misbehavior (by private citizens and/or the police), for example.
In contrast you're generally not allowed to record stuff happening in a private space unless everyone's been informed that this will happen.
This is why you'll see signs saying "Warning - this place is under surveillance" signs on every single door going into a corporation that wants to use security cameras.