I'm not sure where you are getting that I am "unlawfully gaining access to something you did pay for". I paid $30 a month to AT&T for having Data on my Phone and that is what I am doing. I am lawfully and rightfully accessing the data service I pad that company for. At the end... AT&T is trying to charge me $20 a month without offering me anything in return. Yes..I am breaking the Agreement I am not stealing. If you have an agreement with your landlord to No have pets in your apartment and them you get a cat, you and no tell the landlord, that is not stealing, that is breaking an agreement and you are liable for the damage as you said...the same way I am liable if AT&T decided to come after me The point is...i am not stealing anything from AT&T.
On the fair use matter, you are the one who doesn't have a understanding of it, you clearly said that you couldn't make copies for your friends and family of a CD or music or app you bought where is clearly legal to do that (unless you make millios of copies claiming they all are your friends). I know what point you are trying to make, but breaking a TOS is different from stealing.
You said it yourself, thank you! You payed $30 for data on your phone and only your phone and the thing AT&T charges you $20 for is the right to use that data on another device. You say they aren't offering you anything in return now because you are capable of circumventing the system. If non of these apps existed you wouldn't have a choice but to pay the $20 if you wanted to tether.
Now if your landlord said no pets unless you pay an extra $20 a month on the rent then that would be a viable point. If said landlord could prove that you had that cat there for 6 months he could still evict you and you would be legally liable to pay the $120 back to him. Did you 'steal' from him? It called theft by deception and that's what your doing to AT&T if you tether and don't pay the tethering fee. If I blow tollbooth without paying the toll or you jump a turnstile at the subway I didn't 'steal' anything but guess what the cops would do if they see you.
You can't make copies for friends and family under fair use. Fair use is deliberately broad and ambiguous because it covers all copyrighted works and was intended to serve as a base for other more specific copyright laws aka DMCA.