From the Straight Talk Terms and Conditions of Service:
14. UNAUTHORIZED USAGE OR RESALE: The Straight Talk phone is sold exclusively for use by you, the end consumer, with the Straight Talk Service available solely within the United States. Any other use of your Straight Talk handset including, without limitation, activation and/or use on a network other than Straight Talk, any resale, unlocking and/or reflashing of the handset is unauthorized and constitutes a violation of your agreement with Straight Talk. You agree not to unlock, reflash, tamper with or alter your Straight Talk phone or its software, enter unauthorized PINs, engage in any other unauthorized or illegal use of your Straight Talk phone or the Service, or assist others in such acts, or to sell and/or export Straight Talk phones outside of the United States. Improper, illegal or unauthorized use of your Straight Talk phone may result in immediate discontinuance of Service and legal action. These acts violate Straight Talks rights and state and federal laws. Straight Talk will prosecute violators to the full extent of the law. Straight Talk reserves the right to cancel any Service without notice.