Because your BlackBerry will still access the data service. If you launch the browser there is nothing stopping the BB from accessing your carriers data server. You will be charged per KB which is a very expensive way to go.
Another example is if you were to buy a regular cell phone like a RAZR without a data plan. The same thing would occur, if you accidentally hit the dedicated key to media net on the front of the phone, it launches the browser, immediately begins sending and receiving data, which in turn adds charges onto your cell phone bill.
That is precisely why the only way is to buy an unlimited data plan, not one of the limited plans, as it's easy to go over the limit and no way to know when you have.
Or, in the event that you elect NOT to get a data plan, be very careful to not launch the browser, or your email client.
It does not matter if you are with T-Mobile, AT&T, or any other carrier. You are subject to the same situation.
Hope this helps.
Cheers!