Why no personal email? Hmm, that's a good one. My guess is it's one or more of the following:
-- I can forward BES based company email out through a BIS server, thereby completely avoiding any company record of me sending an internal document outside of the company. I can do this with my PC as well, so I'm not really convinced of this one. Data leakage happens (but it can happen invisibly this way).
-- It's possible you will receive some form of a virus/malware/trojan via your BIS email and transfer that to your company computers. It might not be something that effects your bb, but would effect your laptop or other company hw. You don't have to look much further than todays news item detailing how new TomTom GPS devices are infecting PC's they attach to with trojan horses. It doesn't effect the gps display (running linux), but it does effect your PC. Again, I can do this with my laptop also.
-- Some efficiency expert looked at use habits and doesn't want you to be able to sit in meetings looking like you are doing real work when you are really IM'ing a buddy or reading personal email. Nah, couldn't be.
-- Likewise, some admin somewhere detests the fact that you might be having fun, and hates the thought of it. Nah, couldn't be.
-- It's your wifes idea, or the wife of your CSO (even if you or your CSO are woman). ;^)