I suppose if they accept you then you could go ahead until they go international and then you can demand your payment. But bear in mind that if smth goes wrong you may not get your money so don't join big time.
They are wrong in making the inference that only those who have US traffic should join and then later say that you must be in the US to sign up.
whatever the case if they accepted your application then you must be qualified. un less there has been a misrepresentation
Right on the application page, it states:
To be considered for the Yahoo! Publisher Network beta program, please enter your information below. Participants of the program must have a valid U.S. Social Security or Tax ID number, and web site content that is predominantly in English and targeted at a U.S. user base.
So if you have applied, you have agreed to the above statement on the application form. So if YPN accepts the application, they are doing so because you have agreed you met the requirements, although you clearly did not. The misrepresentation was not on the part of YPN.