Why does the FBI have its own Air Force?

Republicans are bashing Attorney General Holder over taking joy rides in FBI aircraft.  Which is a a legitimate zinger, but more interesting is the fact that the FBI has its own aircraft.  Why?  Why cannot FBI agents fly commercial, stand in security lines and get groped by TSA just like the rest of us?   Aircraft are ultra expensive to own and operate.  Corporations are thinning out their aircraft under stock holder pressure.  As far as stockholders are concerned, corporate aircraft just suck up money that could otherwise go to dividends.  As far as this tax payer is concerned, FBI aircraft just suck up money, they don't stop crime.   Yet another place to do a little sequestration.