Thinking about this, it seems like Google and Yahoo have been really envying what is effectively Facebook's captive audience. With Facebook, you sign up for "social networking" is essentially an undefined product whose provider has undefined obligations to you.
Email involves some explicitly or implicitly understood bounds and so doesn't let the provider sell it users to the same degree - well, unless the email provide just flagrantly violates their implicit obligations - and so the temptation is greater and greater.
Thinking about this, it seems like Google and Yahoo have been really envying what is effectively Facebook's captive audience. With Facebook, you sign up for "social networking" is essentially an undefined product whose provider has undefined obligations to you.
Email involves some explicitly or implicitly understood bounds and so doesn't let the provider sell it users to the same degree - well, unless the email provide just flagrantly violates their implicit obligations - and so the temptation is greater and greater.