That’s not true. Oh sure, the secret service will protest, but they still know who the boss is!
There would be nothing wrong with actually planning to stop at the birthday party, as I said I like it that he did that. I just don't think that Bush would do something like demand to stop at an event like this without getting the thing signed off on by the SS.
My take on this is that that the SS would quickly weigh situation up make their concerns known the President and their aids, add what they can do to secure the area quickly and then it would be the President and his aid’s decision but bottom line it is the President’s call.