Even if most people don't have the technical chops spy on traffic themselves, I can imagine a world where there are companies that provide such things as products / services. (Probably there are already a few companies like this, I haven't checked.) For example one might be able buy a gizmo with an antenna that listens in on people's wifi and mobile connections. If end to end encryption was banned, I'm guessing that spying on that non-encrypted traffic would take about $50 and an afternoon of setup, and would not require any special skills.