I think that the difference between your conversations and pizza ordering on an analog transmission and on a digital one WRT 4th amendment protection should be zero.
This isn't about analog v. digital. With an unencrypted cell phone transmission you are still paying for service and not just anyone is allowed on the network. Your phone has an ID that tells the carrier who you are so they can allow you to use the network and to bill you for service.
With an unencrypted wifi hotspot, it is open for anyone to use. Perhaps you may need to purchase something and enter a code in your web browser (although this is more and more rare), but by and large, the wifi owner doesn't care who you are. They are providing a service to anybody, not specific individuals.
Put it this way, with a car analogy. If I lend my car to a friend and charge money, there's no big deal. But if I advertise and let anyone rent my car, I am suddenly operating a business and a whole different set of rules and laws apply.