There are a few reasons...one, the system does notify you at least three times via text that it is going to happen, and to call 611. Most people ignore that. Another reason is that if you don't have a data plan, then you are going to be charged at the $2 per MB "pay per use" charge...so instead of $30 for 3gb, you could easily end up with $6000 data bill. Also, buried somewhere in your contract is a clause that we can change your plan if your usage will negatively impact the network...of course there isn't really a straight-forward reason is to how a smartphone without the right data plan does this, but that's the way it's written. Right here (http://www.wireless.att.com/cell-phone-service/legal/index.jsp?q_termsKey=wirelessCustomerAgreement&q_termsName=AT%26T+Wireless+Customer+Agreement§ion=termCommitChargesBillingPayment&q_subTitle=TERM%20COMMITMENT,%20CHARGES,%20BILLING%20AND%20PAYMENT#canAttChangeMyTermsRates) you agreed that AT&T "We may change any terms, conditions, rates, fees, expenses, or charges regarding your Services at any time. " If you have an LTE smartphone without an LTE plan you will probably only connect at 2G speeds.
That contract also states that specific data plans are made for each class of phone. And the very first clause states "Your Agreement begins on the day we activate your Services and continues through the Term of Service specified on your Customer Service Summary ("Service Commitment"). AT THE END OF YOUR SERVICE COMMITMENT, THIS AGREEMENT WILL AUTOMATICALLY RENEW ON A MONTH-TO-MONTH BASIS."
There is a huge difference between this agreement and the contract time of the phone itself...your "contract" with the phone may be up, but the agreement never expires. Think of it like renting a house...once your lease is up, you still have to pay the rent if you want to stay there. It's the same with AT&T...the phone's contract is up, but your agreement with AT&T is still valid until you quit using the services.