Why i am billed for whole month even though i used the service for 4 days ?

I ordered a Wireless Postpaid Service starting from June 9. And i got my first bill on June13 only (amount $96). I used the service from June 9 to July 13 (1st billing cycle) and disconnected it.
But I again got the new bill from June 13 to Jul 14 (2nd billing cycle) (amount $93)

I called up AT&T and asked why i got billed for the whole month($96) even though i used the service for only 4 days? (June 9 to June 13) in the first billing month.

They said that you billing cycle starts from 13th of every month and even though you used only 4 days of service , you would be billed for the whole month.

I dont understand this where it is written in Terms and Service that a person even though uses the service for 4 days, he/she has to pay for whole month ???