Discontinued cell service 1/3 rd into month, Att wants payment for month

Doesnt matter that you say you will only pay prorated charges. You signed saying you would pay the full last bill when you started service. What you signed was a written agreement between both parties, where you saying you will only pay prorated charges wasnt an agreement between both parties.

Regretfully,  very few people (if any) read the ToS (Terms of Service) that they agree to when they accept their service with a cellular carrier. Those ToS explain things like this to the user so that they understand what they are expected to comply with in their service from the provider.  The fact that the final month service bill is not pro-rated, because your service is not prorated.  If you keep the service and use 100% of the plan in the first week, but use zero for the rest of the month, you are not charged overages.  If you use nothing for the first 3.5 weeks, then use everything in the next to last day of your cycle, then you pay no overages.  If you cancel your service how is AT&T to know how much data you would have used in the portion of the month had you not canceled?  They can't so they can't properly pro-rate your service.  It's just like the "Flexible Spending Accounts" for medical expenses.  Some customers suffer from the "use it or lose it" status,  others benefit from the fact that is it 100% available on day 1, and if you leave early, you don't have to pay it back.

Exactly one carrier prorates service on final bill.  Google Fi.   

No other major carrier and zero prepaid carriers prorate your bill.