[The problem is that while the rendition thing is illegal, reading your email as it crosses the border is actually perfectly legal. (And keep in mind the ideas of the Fourth Amendment are still regarded as deliriously weird in most countries of the world, even in Europe.)
Actually, Common Law provides for guarantees against unreasonable search and seizure in all nations based upon it. A number of them have a similar clause in their Constitutions.
What non-Americans regard as delirious about the 4th Amendment is the case law application of it, which evidence introducing absolute certainty of guilt is excluded from court entirely without discretion because of a rights violation. Most other countries do a balancing act on the issue, wherein if non-admission of evidence, even if illegally gathered, would bring the administration of justice into disrepute (Canadian wording), the evidence is in.
Opening people's mail if it crosses borders is an old and oddly respected practice. It was endemic to Europe for hundreds of years, and literally every such letter that crossed German or German-occupied soil in WW2 was opened, yet you'll notice it wasn't charged at Nuremburg because it wasn't against any law. This is because the right of a country to control their borders for goods, people, and communication is considered sacrosanct, and the first step to control is knowing what's actually coming in and out. The only communication exempt from that is communication which passes under diplomatic seal.
True, and legally justified under international customs laws. Correspondence receives no protections beyond any other goods when it crosses the border - it's subject to inspection, same as any other good. This is why Customs can and will search your papers, your electronic devices, and your computers.