ACV, I'm sorry if you are confused as to what you are paying for, but I'll try to better explain what the $9.99 plan covers so that you do not continue to pass incorrect information.
The $9.99 plan for T-mobile, as per the TOS (terms of service) is for unlimited email use only. The plan that has been put into place was not created by Research In Motion, who is the host behind all BIS/BES service. T-mobile created this plan by adjusting a regular BIS (blackberry internet service) plan to not send down the browser service book. Since this plan isn't an actual built-in option by RIM, there are obviously holes to this. One of these holes is that despite it being an email only plan, you can still send/receive attachments and 3rd party applications can still utilize the BIS connection. Yes, email is data, but emails containing only text use very little data. In comparison, it would take a thousand emails to even match the amount of data transferred for one high resolution pic. Despite it saying you have unlimited data online (which means yes, you can send as many emails as you like), if you abuse the terms of your service, which states for email use only, they have the right to invoke charges to your account for any activities outside of the said agreement.
T-mobile is pretty relaxed with most of its rules and isn't looking to stick customers for every penny. If they see say 10mbs of data used, it's pretty clear that you obviously weren't just sending emails and if they see enough users abusing the hole, they can decide it's time to charge and you will not have a leg to stand on since you ignored the agreement.
I hope this has cleared things up, particularly terms of service.