But a cryptocurrency-backed exchange does not provide independence of a third party; so a in the pretty implausible hypothetical that you'd ban systems so gratuitously inefficient, you could make the argument that peer-to-peer transactions are OK, but that case is harder to make for exchanges.

I mean, not that this kind of ban is likely to happen; it's probably not even a good idea (unless we can be much more specific about what counts as wasteful, and what counts as comparable existing technology, and have a way to make that simple, please no patent prior art search 2.0 nonsense).