Any violation by the foreign air carrier of applicable provisions of Subtitle VII of Title 49 of the U.S. Code or of orders, rules or regulations issued thereunder, or of the terms, conditions or limitations applicable to the exercise of the privileges granted by the permit shall constitute a failure to comply with the terms, conditions and limitations of such permit: Provided, That upon a showing that a violation of a provision not mandatorily prescribed by law resulted from the observance by the holder of an obligation, duty or liability imposed by a foreign country, the Department may excuse the violation.