Your almost to the right question: If we, the citizens, are handed a circular butt-covering load of excuses as to why one agency can not give another one information that is necessary for proper and duly passed law enforcement shouldn't we citizens conduct our affairs along the same lines?
Seriously though, many posters have noted the President, Congress, heads of the respective IRS and Social Security Administration, the Supreme Court and ultimately US possess the power to make these agencies cough it up and provide this information to the FBI (note I omitted Homeland Security - too rife with corruption and agendas). No agency has immunity from the citizens they supposedly serve except those handling critical national security information. The last I heard the SSA and the IRS are not remotely dealing with business that's in the CIA and NSA domain.
“...note I omitted Homeland Security - too rife with corruption and agendas...”
Homeland (In)Security should be abolished. As fas as US is concerned, it may not be too soon for US to use the power that US has. Complacency makes US forget who US is and what power US really has.