As I understand it, the port that USB is implemented on for the Pi machines is actually meant as an OTG port, made to act as a host by the drivers. On models with more than one USB port, there's a hub+ethernet chip. On the single-USB boards, they can actually be configured to act as OTG devices.
The whole situation means that OTG is unavailable on most models, models that have it only get one USB port (so OTG/host is an either-or proposition), and for the multi-USB+ethernet models, performance over those interfaces is worse than you'd expect (if I remember, BBB gets several times the transfer over USB and network, all at lower CPU usage, right?)
