Well, maybe it's more accurate to say that it's impossible to consent because no consent is required to take someone's picture in public. People generally exercise control by choosing not to be in public. You could say that having your picture taken is part of the terms of service of using public space. When you agree to something, you also agree to all of the consequences. Just because they are implicit doesn't make them invalid.

Don't get me wrong, I don't particularly like having my picture taken without my explicit consent. In the end, consent is all rather arbitrary because it's not like you can choose not to live in human society on Earth.