Secret Service let it happen, but the planmakers were higher up!
They have done it before at Dealey Plaza on Nov 22, 1963 when they pulled the agents off the back of the car Kennedy was riding in.
First of all, standard operating procedures for the Secret Service were to place agents at rear of the presidential limo and along the sides. at Love Field where President Kennedy and company started their tour of Dallas, two Secret Service agents took station at the back of the car, but were ordered off by Secret Service Agent Roberts, with no objection from Kellerman.
THE U.S. SECRET SERVICE MURDERED JFK (thesecretservicejournal.blogspot.com)