Look at their web site, the tethering options are gone. If you read the terms of service when you start the hot spot, it basically say's this should be used for checking email, light browser use and so forth. It also states that it is not to be used to stream video, PTP file sharing, downloading music or streaming television. If they find you in violation they will terminate service. There is no mention about fees.
Like others have said, I think if you use it sparingly you will be fine. If you are on You Tube and watching movies in HD, you will throw a flag up.
For e, I use it when travelling so the kids can get on the Ipods and surf the web.