The "reasonableness" standard is objective not subjective reasonableness. It's objectively unreasonable to think that your data in say gmail, which is stored in a form accessible to many people you've never met, and is in fact data-mined by the host, is somehow "private."
If I keep an unlocked box full of papers in my neighbors garage, and he sometimes rifles through them looking for something interesting, can I say that box is "private"? Maybe if the box is locked and only I have the key (I.e. Client-side encryption) but not if I send it to him locked but give him the key (I.e. HTTPS). Again, it's not about legal technicality or technical technicality. It's about whether the information is actually "private."
If I keep an unlocked box full of papers in my neighbors garage, and he sometimes rifles through them looking for something interesting, can I say that box is "private"? Maybe if the box is locked and only I have the key (I.e. Client-side encryption) but not if I send it to him locked but give him the key (I.e. HTTPS). Again, it's not about legal technicality or technical technicality. It's about whether the information is actually "private."