> * My employer owns my laptop and the network it's connected to: of course it has every right to inspect its own property.*

Careful there: there's a difference between capability and right. Many legislations forbid employers to put surveillance cameras in the bathroom for instance. There is a level of privacy employees can legally expect.

E-mail is similar: if your employer intercepts your e-mail just because you read it at work, it is likely a breach of correspondence secrecy (in France it would be). This likely applies even if you're at fault for using the company's resources on personal matters.

While "don't trust your employer's network" is elementary operational security everyone should be taught at school, 90% of the population don't know what computers can do, let alone how they work. Because of that, their expectations are social, not technical.