Look closely, two SS men can clearly be seen with their backs to the President, as others are no doubt watching all the hands. Yep, even the manhole covers are checked out enroute.
Our Presidents are isolated enough as it is, and there are times when mingling with the people is necessary and desirable.
Bet the rent, this was more than likely completely unannounced and totally unscheduled, that way nothing leaks. There is a possiblility that some of his handlers up front scouting several miles ahead saw an opportunity, and made the suggestion and quick security plans were made, but I'd bet it was completely unannounced as the SS briefed the President early on, that the only way possible to mingle with the public would be to do so in this manner.
We can't remove all risk from our lives, or theirs.
No, but the SS does a pretty good job of eliminating *most* threats to the POTUS.