Many photos are taken at bars or parties of people who are acting funny but that is not right if they do not want the picture taken because their futures can be affected by someone else unknowingly taking a picture of them. Finally an example of a situation where it is illegal to take a photo but in my opinion is ethical is in a place like congress or many courtrooms where it is not allowed. These are public spaces and therefore it is ethical for these places to be documented by the said public.
Overall there are a lot of great examples that back up the claim here