I wouldn't worry. Although 81 gb is a lot, there are people who use more. I wouldn’t worry until they contact you in some form, which will likely never happen
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We can, without notice, limit, suspend or end your Service or any agreement with you for any good cause, including, but not limited to: (1) if you: (a) breach this agreement; (b) resell your Service; (c) use your Service for any illegal purpose, including use that violates trade and economic sanctions and prohibitions promulgated by any U.S. governmental agency; (d) install, deploy or use any regeneration equipment or similar mechanism (for example, a repeater) to originate, amplify, enhance, retransmit or regenerate an RF signal without our permission; (e) steal from or lie to us; or, if you're a Postpay customer, (f) do not pay your bill on time; (g) incur charges larger than a required deposit or billing limit, or materially in excess of your monthly access charges (even if we haven't yet billed the charges); (h) provide credit information we can't verify; or (i) are unable to pay us or go bankrupt; or (2) if you, any user of your device or any line of service on your account, or any account manager on your account: (a) threaten, harass, or use vulgar and/or inappropriate language toward our representatives; (b) interfere with our operations; (c) "spam," or engage in other abusive messaging or calling; (d) modify your device from its manufacturer's specifications; or (e) use your Service in a way that negatively affects our network or other customers. We can also temporarily limit your Service for any operational or governmental reason."
The question of whether or not the USB tethering is a violation of the terms of service is something that's somewhat up in the air. Most Android devices have the capability to tether via USB built natively into the device, but normally carriers block this or route it through a portal where they ask you to pay for it (like with the wireless hotspot). Since this isn't the case with the Bionic, it definitely leaves a bit of a question about it.
Current lineup: Galaxy Note 2, Galaxy Note 8.0, MotoActv
Previous smartphones: Galaxy S3, One X, Droid RAZR MAXX, Galaxy Nexus, Rezound, Galaxy Note, iPhone 4S, Droid Bionic, Stratosphere, Droid Charge, Vitality, Droid Incredible 2, Thunderbolt, Palm Pixi Plus, Droid 3, Fascinate, Droid Incredible, Droid Pro, Droid 2 Global, Droid X, Droid, Treo Pro, Touch Pro 2, Touch Pro, Q9m, XV6800, XV6900
Previous tablets: Transformer TF300, Evo View, Transformer Prime, Transformer TF101, Thrive, gTablet