Насколько я понял, фирмы вынуждали использовать именно эти реитинговыe агенствa для некоторых вещей:

http://en.wikipedia.org/wiki/Big_Three_(credit_rating_agencies
From the mid 1970s until early 2003, the Big Three were the only "Nationally Recognized Statistical Rating Organizations (NRSROs)" in the United States

http://en.wikipedia.org/wiki/Nationally_Recognized_Statistical_Rating_Organization
A Nationally Recognized Statistical Rating Organization (NRSRO) is a credit rating agency (CRA) that issues credit ratings that the U.S. Securities and Exchange Commission (SEC) permits other financial firms to use for certain regulatory purposes