Why shouldn't content providers be free to monitor usage if it's legal and easy to do? Would you next start to argue SaaS providers shouldn't have any Web server logs at all since it's a way of "tracking" users?
If a piece of content is delivered to me via the mail, I should be able to open a cached version as many times as I want without any request to the remote server.
And the cached version can be built for me by my mail system, which by ALWAYS fetching the resources protects me.
As a question of what SHOULD happen, I thinkread receipts should be voluntary.
Agreed, and not even the server has to do that, any good e-mail client could (as it seems Gmail is now starting to do).