Not sure completely what is the reason unless there are something different with both your plans.
My daughter ran on a Blackberry 9700 Bold, then got tired of if even though she still had a contract. I gave her my Samsung Galaxy S, which by then was updated to Froyo. The phone worked including mobile internet.
Note, a Blackberry data plan consists of two things: the mobile internet and within it, the Blackberry Internet Service that pushes email and social networking notifications (BBM, Facebook, Twitter) to the phone. The same plan should work on any other smartphone using mobile data, and in fact, it would not affect separate systems of push notifications, like GMail, Hotmail, Facebook or Twitter, because all these go through this regular internet.
In some countries, there are BIS only plans that are so cheap because they are BIS only but don't have regular mobile Internet. This plan lets you have push email, Twitter, BBM and Facebook notifications, but does not have mobile browsing or access to the other web using apps. For many people, these plans are more than enough, and its a reason why Blackberry is doing so well in emerging countries.
Now if you have such a plan and you put the SIM on an Android smartphone, you get voice and text only but no regular mobile internet. You can still use wifi though.