I don't think the Whitehouse even had a fence around it in those days. (If it did, it wasn't much of one.) In the OP photo, it's interesting how folks are on rooftops, nearby parked cars with the doors open, etc. Definately different times.
However, I don't think the change was as abrupt as it may look like it was looking back now. Recall that President Ford was walking through a crowd in California when Squeaky Fromme attempted to do him in with a Colt 1911. She had it trained right on him from a few feet away. She never got a shot off since she had (fortunately!) neglected to educate herself the proper operation of said handgun. To think that a nit-wit like that came so close to assinating the President.