You should expect to be safe from that detective in your own house, because we’ve drawn a very clear distinction—that detective is breaking the law by snooping on what you do in your own private residence. If the police wanted to discover what you were doing in your own home, they’d have to get a warrant in order to not be breaking the law by doing that (“warrant” literally being defined as “a writ making something that’s normally illegal to do, temporarily not, for you, in this situation.”)
Whereas, neither the private detective, nor the police, would be breaking the law by snooping on you in the public square. So either/both would feel empowered to do so, because society does not think such an action is worthy of punishment.
That's pretty circular logic. There are feedback loops between expectations and law, but that doesn't mean the status quo is always correct. The nature of how much surveillance one person can perform has increased by many orders of magnitude. It's worth rethinking how we limit public surveillance.
Whereas, neither the private detective, nor the police, would be breaking the law by snooping on you in the public square. So either/both would feel empowered to do so, because society does not think such an action is worthy of punishment.