I believe my statement is more broad than your clarification and so I did say that, but for the sake of the argument, yes, that is indeed what I'm asking for, and I believe that's entirely reasonable. If you have the ability to send encrypted data over any particular communication channel in such a way that no untrusted third party can ever decrypt it (let's assume that it's possible), then I think that should be entirely lawful.
I'm not saying that all communication channels are designed in such a way as to make that possible, but for those that are, I believe that's completely ok.
There is a long tradition of disallowing encrypted communication on channels that can be (are meant to be) public and snooped by the public.
See licensed Ham radio use. There's no technical restriction or necessity against (well, necessity is up for debate), but there is a legal prohibition on commercial or encrypted transmissions; I read recently that it was being discussed to change this. I guess it would have been reported on HN frontpage if this had gone through.
The argument as I understand it is that Ham bands are a shared, public resource (commons) and that their use is strongly regulated by the FCC (and presumably some international body I haven't heard of); if your communications were encrypted, you would be free to violate the rules and engage in commercial activity, which would likely make the band a lot more popular, polluted, and less available as a limited resource for amateur licensed users. It's called broadcast for a reason.
This is obviously far from the situation on the internet, but it's a fun thought experiment to imagine the global internet with similar structure of mandatory licenses for use, and without any encryption or commerce.
I'm not saying that all communication channels are designed in such a way as to make that possible, but for those that are, I believe that's completely ok.