Every country has its own laws regarding entrance by non-citizens. Please check the specific laws governing the country you are visiting. It is your sole responsibility to obtain all necessary documentation prior to your departure. Regardless of the ticket you purchase, applicable immigration laws apply.
Please note that there is a new U.S. regulation in effect that requires citizens from certain countries to obtain a visa if their flight is stopping over in the U.S., even if it is not the final destination.
In addition, citizens from waiver-countries should be aware that in accordance with the U.S. Department of State, travelers entering the United States under the Visa Waiver Program must have a machine-readable passport. Any traveler without a machine-readable passport will be required to obtain a visa before coming to the U.S.
For more information on U.S. visa regulations, visit: U.S. Department of State.