For the same reason that sending me a postcard doesn't entitle you to enter my house and take an inventory of my fridge. To get that information about a user, you should need informed consent, not just a marketer's wistful greed. That same "why not?" attitude is exactly the same bullshit user-contemptuous line of thinking that the NSA and the FBI use with their "intercept it all and let God sort it out" eavesdropping schemes.

If I choose to visit a website hosted by a server operated by someone else, that server is involved and can do whatever it wants.

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.

Web server logs are okay. Any javascript-based tracking and cross-site usage aggregation is not.