I believe the reason is that RIM actually runs its own BlackBerry email/IM service because they have their own network overlayed on top of the carriers. It is encrypted as well, that's why FBI bought 20 thousand BBs last year so that NSA can't see the email traffic, because not even NSA can decrypt the 1024 bit encryption.

Also, remember reading something about why India was reluctant to let BB in - for the same we-cant-see-the-bb-traffic reasons.