All carriers have vaguely worded sections that say to the effect "we may terminate service at our sole discretion...." AT&T is just stating some specific items that can cause you grief if you "don't play nice."
For example from
Verizon Wireless' Customer Agreement:
Our Rights to Limit or End Service or This Agreement
...WE CAN, WITHOUT NOTICE, LIMIT, SUSPEND, OR END YOUR SERVICE OR ANY AGREEMENT WITH YOU FOR THIS OR ANY OTHER GOOD CAUSE, including, but not limited to: ...(ii) if you... (e) interferes with our operations;...(h) uses the service in a way that adversely affects our network or other customers....