It's called the U.S. Postal Service because it is a service, not a corporation. Who decided the Postal Service must be profitable? Do other government agencies, like the Defense Department and the Department of Education, have to turn a profit?
Benjamin Franklin was the first postmaster general, and the Post Office has effectively served the American people for 237 years. But now it is under attack because, gasp, it's losing money. No wonder: The Republicans in 2006 made postal workers pre-fund their retirement 75 years in advance, making it nearly impossible for the USPS to make a profit.
Let's be honest: Conservatives want to privative the Postal Service, home to two of the nation's largest unions. If Congress insists that the USPS be profitable (and it shouldn't), then the solution is simple: Raise the price of stamps a few cents.