It was an example, not a premise.
Okay, well let me put this to you.
Almost all of my data goes through a VPN. P2p, http, streaming hulu/netflix etc. - almost everything other than games. Since it is on an off port, what happens as soon as I am shaped? How can I be guaranteed that it wont be considered "a large download".
If most of my bandwidth goes through this off port, and it is heavily encrypted and the ISP physically can't tell what the traffic actually is, how can they guarantee my browsing or streaming wont be affected? I happen to use the same port for fetching p2p stuff too. So what now?
The easiest way is to zero-rate my data during off peak. I wont be using lots of bandwidth through this specific pipe during the day - but if it is encrypted, how can they possibly tell the difference?