I know it's code (refer to NEC 250.24(A)5 and 250.32), but I don't understand why you can't use a bonding jumper to connect the ground bus and neutral bus in the subpanel? I've been told it's because then you'd have current on the grounding conductor. In my attempt to understand this, I was thinking maybe it's a more of an issue of having parallel neutrals... but if they're grounded on both ends, how would that (not) work? Possible safety issue: if one of the grounds becomes disconnected, then you have a true parallel neutral... is that a viable or better explanation, or is current on the grounding conductor the real issue?
If so, then why isn't current on the grounding conductor(s) from the main panel a problem? I don't see the inherent difference between a subpanel and a main panel. Ultimately this question is, why bond neutral and ground in the main panel, but not the subpanel (besides NEC said so)? Why is the neutral from the main panel to the subpanel inherently different from the service neutral to the main?
edit- you may also refer to: How to properly ground a subpanel in detached building? for a relevant picture and because it is a related thread.