Reasonable Expectation of Privacy | EFF Surveillance Self-Defense Project: "The Fourth Amendment only protects you against searches that violate
your reasonable expectation of privacy. A reasonable expectation of privacy exists if 1) you actually expect privacy, and 2) your expectation is one that society as a whole would think is legitimate. This rule comes from a decision by the United States Supreme Court in 1967, Katz v. United States, holding that when a person enters a telephone booth, shuts the door, and makes a call, the government can not record what that person says on the phone without a warrant. Even though the recording device was stuck to the outside of the phone booth glass and did not physically invade Katz’s private space, the Supreme Court decided that when Katz shut the phone booth’s door, he justifiably expected that no one would hear his conversation, and that it was this expectation — rather than the inside of the phone booth itself — that was protected from government intrusion by the Fourth Amendment. This idea is generally phrased as "the Fourth Amendment protects people, not places.""
Furthermore, it is important to note situations in which the Fourth Amendment does not apply--here's one (read the full article at the link above for other situations):
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Public places. It may sound obvious, but
you have little to no privacy when you are in public. When you are in a public place — whether walking down the sidewalk, shopping in a store, sitting in a restaurant or in the park — your actions, movements, and conversations are knowingly exposed to the public. That means the police can follow you around in public and observe your activities, see what you are carrying or to whom you are talking, sit next to you or behind you and listen to your conversations — all without a warrant. You cannot necessarily expect Fourth Amendment protection when you’re in a public place, even if you think you are alone. Fourth Amendment challenges have been unsuccessfully brought against police officers using monitoring beepers to track a suspect’s location in a public place . . . "
Hence, security cameras or webcams in public places which also provide a deterrence to crime.
