In all this what's legit content or not, we should remember the intended application of BIS is 'on-device', i.e. you can do what you want, within limits, ON the device. If you dock your BB, it switches to the internet APN.
I'm not saying this completely clears up the grey areas (it does not), but if you're downloading 300GB of stuff, it's pretty clear you're not using that on the device.
RIM used to manage this well with file size limitations (they still limit things like e-mail attachments, etc.). Services like YouTube and other streaming services also switched to the internet apn. But lately people have been doing clever things, like the HTTP gateways for torrent downloads which clearly is going against RIM's intentions for the service.