Everyone is (rightly) taking issue with your mailbox example, but the substance of your point is correct. The Fourth Amendment doesn't require that investigators get a warrant to look at information that you've already handed over to a third party. That's because the Fourth Amendment suppression-of-evidence rule protects citizens' privacy interest. If he has already made the information non-private, then investigators can't be said to have violated his expectation of privacy by accessing it.
Note that the rule doesn't require suppression of just any evidence obtained illegally -- only evidence obtained in violation of the defendant's constitutional rights (and, in particular, his right to privacy). Relatedly, note that the suppression rule is not actually written into the Fourth Amendment. It is a judicial creation to deter particularly egregious violations of the Fourth Amendment. http://en.wikipedia.org/wiki/Exclusionary_rule#Limitations_o...
>The Fourth Amendment doesn't require that investigators get a warrant to look at information that you've already handed over to a third party. //
You're right, if they ask that third party to see it. But if they don't ask, they hack in, then they're breaking the USC that protects against unauthorised access to computer systems [18 USC 1030?].
The third party usually will have a legal requirement not to divulge your info without a warrant (except perhaps under conditions threatening national security). I'm not sure on this for USA, most countries have laws protecting personal information; your contract binds the company to not disclose information but a warrant supersedes that ... but presumably there's not a general regulation forcing companies to disclose information to authorities who merely request it as opposed to getting a warrant. That would be most undemocratic.
Thus under rule of law, despite the Fourth not applying, the FBI would still need to get a warrant before either hacking in or making the company disclose the information, no?
Like, they can open your mailbox as the postal service own it, but if it's locked then they'd need to get the key/authorisation from the owner (or they'd break the law, breaking and entering or bypassing security or somesuch). If the owner says "no" then they'd need a warrant (legally speaking) to force the owner to open the box and take your post out. If they just break in and take the post it's not a Fourth Amendment violation against you, AFAICT, but it's still unlawful.
IMO the evidence should still be used but the fact of the evidence being obtained means that if submitted by the state it's also evidence that a person working for the state committed a crime. Rule of law means that person should be tried as should any co-conspirators. But this last para is just my idea of democratic responsibility and not how things are.
Note that the rule doesn't require suppression of just any evidence obtained illegally -- only evidence obtained in violation of the defendant's constitutional rights (and, in particular, his right to privacy). Relatedly, note that the suppression rule is not actually written into the Fourth Amendment. It is a judicial creation to deter particularly egregious violations of the Fourth Amendment. http://en.wikipedia.org/wiki/Exclusionary_rule#Limitations_o...