The word "unlimited" is unfortunate, and most carriers are guilty of mis-using it. But-the previous posters are right, bandwidth isn't infinite, and limits (within reason) make the product more accessable for everyone.
As for charges for tethering are concerned, in a Free Enterprise society, companies have a right to charge for their services. As consumers, we need to make choices that best suit our needs. Is the best choice for each individual always there? No. This is why a competitive market is a good thing.
1st Step in Troubleshooting: Do you have a BlackBerry Data Plan?
2nd Step in Troubleshooting: Pull the Battery.