In US privacy (case) law. "Reasonable expectation of privacy" is a specific term that determines whether it's okay to record things. There is a lot of case law

In US privacy (case) law. "Reasonable expectation of privacy" is a specific term that determines whether it's okay to record things. There is a lot of case law around what does and doesn't imply a reasonable expectation of privacy. Hence people are using the term in a specific legal way, rather than saying "I don't think it is reasonable to expect privacy here", they are saying "I think legal precedent would make a judge rule there is no 'reasonable expectation of privacy' here".