That's part of it. But there are other reasons. A company-issued Blackberry, using a company provided RIM server, can control the BB remotely. If it is stolen or you leave the company they can disable the BB from the central server and erase any email stored on it.
However, some of the business community HAS adopted the iPhone. I use mine primarily for business. It's primarily very large companies who closely control access to IT resources who don't like the iPhone for several reasons: The control issue, the fact that they would have to train support staff, the fact that ANY change to the IT infrastructure requires months of discussion and review, fear of the unknown. Now if the CEO wants an iPhone they may move quickly to support it, but if the average worker or middle manager wants it there is little chance of making it happen.