PRISM doesn't involve data from US citizens' accounts. Please review what PRISM is before claiming it is illegal or unethical. It is neither.
If you are incapable of reading the documents yourself: if the company receiving the wiretap order has any reason to believe that an account belongs to a US citizen or a non-US citizen living in the US, they challenge the order. No warrant is needed in the US legal system or that of any other country in the world to wiretap a foreigner who is outside of the country requesting the data. The fact that the US requires a court order is rare, and by all accounts, including Snowden's documents, that procedure is followed.
If you are incapable of reading the documents yourself: if the company receiving the wiretap order has any reason to believe that an account belongs to a US citizen or a non-US citizen living in the US, they challenge the order. No warrant is needed in the US legal system or that of any other country in the world to wiretap a foreigner who is outside of the country requesting the data. The fact that the US requires a court order is rare, and by all accounts, including Snowden's documents, that procedure is followed.