You might have to use another app that hides your activities such as PDANet or Clockwork Tether. Unfortunately though they do have free versions, they have limitations that might get annoying if you don't pay for the premium service. Another option would be a custom rom that has the tethering provisions removed already. Oh yea, if you are still on the stock rom, check for some kind of tetheringprovision.apk in /system/app. If there is anything like that in there, if you freeze it or rename it, that might help as well. Just hard to be more concrete with suggestions as carriers can have their own ways to trip up tethering usage vs others and it might just be a glitch, or they have something set up to make this a pain.
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