This isn't about security or privacy. It's about social norms. Most people don't want to offend people, don't want to record people without their consent, and don't want to make people feel uncomfortable because they think they might be recorded. Establishing a social norm that people should outwardly notify others when they're recording will mean fewer people will record people who don't want to be recorded because of social pressure. Certainly, creepers who want to record others secretly will do so, but they will have violated social norms for having done so, and won't have any defense since they will have had to deliberately obfuscate their activities to do so.