My underlying thoughts are as follows. Yahoo is the target in this story, so I'll use Yahoo as the placeholder for any company that deals with user data in that capacity.
When you send an email, it shows up at a destination. You send it from your mail provider, "magic happens", and it shows up at the destination. By definition, the text that you send it what is received.
By sending your text with who you want it to go to, you are relinquishing control over your data. Now, Yahoo can read it. And any mailserver in between. And the destination mailserver can as well. You also give up a form of copyright- because the basis of this transmission means that the same message may appear multiple places.
With the USPS, we pay postage to send data and packages. And there are stringent safeguards in place for the USPS. We have no such data safeguard laws in the US. European Union is different (I don't know the laws there). But whatever data I send to some private org, can be used and reused in whatever ways that are bound by their privacy policy (which they write), and whatever contracts they abide by (PCI DSS, HIPAA, FERPA, etc).
Even though this part isn't logical, whenever I don't pay for a service, I automatically assume that I'm paying with my data. I guess when you're cash-poor, it's a good trade-off.
