It's just that streaming Pandora or Sirius is a pretty constant data stream. My kid uses Pandora, and Yahoo Messenger on his phone all day, every day. His data usually averages 7GB to 12GB per month which is about what I use when tethering several times a week. There are lots of free streaming media options these days. Your phone most likely came with YouTube. If you were to use it all day, every day, had your phone pulling your emails every 15 minutes, were using google maps, etc... you'd see really high data amounts. And that's just using the software that the carrier included with the device.
Since streaming media has become so popular, the carriers aren't doing much about the high data use of their customers. You can rack up a huge amount of data doing stuff that PDAs are made to do without tethering. So long as you have an unlimited data plan, it's most likely going to be "all good".
I had 1 month with Verizon that was almost 25GB and they didn't say a thing. Highest I've had with Sprint so far was 18GB. (I use limewire a lot for audio and video, with some video files as big as 1.5GB each) So it's really unlikely that you'll be "caught" tethering.
Notice I said unlikely. I would hate to see you be the first person to get nailed for it, then me feel bad about telling you all this. All I can say is I tether every day, use gobs of data, nobody on Sprint's end seems to care.
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