I thought they were responsible for protecting the President, etc., and fraud investigations were the purview of the FBI, Justice, etc?

The Secret Service was originally established to safeguard & protect U.S. currency from counterfeiters.  I saw that on a very good episode of "American Greed" about a group of guys in L.A. who were making excellent counterfeit currency using a computer & printer.  The Secret Service also investigates financial crimes & protects our financial payment systems in addition to its protection of the president, vice-president, etc.  It continues those responsibilities even though it's now part of DHS & not the Treasury Dept.