Probably because their official stated purpose has been “conducting investigations into currency and financial-payment crime, and protecting U.S. political leaders, their families, and visiting heads of state or government.”[0] Especially since US secret service was originally created with the former purpose, as protection of political leaders was added to their list of responsibilities only later on.
filoleg|1 year ago
0. https://en.wikipedia.org/wiki/United_States_Secret_Service