A better analogy would be FedEx selling your incoming and outgoing addresses and package weights to third parties, not necessarily the contents of your packages.
It sure seems like they could. For most people most of the internet is still insecure.
Or slightly worse, they could get browser vendors to include their CA (or pass legislation to force this).
What prevents this from happening?
In mobile, where the carrier controls everything (the network, the OS) it's not unlikely this is already happening.