Also the White House is always stopping the tours — for any reason. In fact, just show up in Washington and see how long it takes to get a tour of the White Hut. (And there are no self-guided tours at the place.)
You are partially correct in that they may cancel the scheduled tours at any time, for any reason, but there have been no "walk-up" tours for a long time now. You have to request a tour through a Representative or Senator's office approximately a month in advance, submit your info to the SS, and if approved, show up at your appointed time. Also, all of these "tours" are self guided, but there are Secret Service in each room to answer questions and point you in the right direction to proceed through the house.