The President of the United States is the worst perpetrator of violence in the world.  I think it is only appropriate that the occupant of that office stay away from protests against violence.  People who live in glass (White) houses, and all.

Furthermore, it's not as if the President would be paying for such a trip out of his own money.  It would be paid for with money taken from his subjects, like everything else the Presidency does.  It's best the he cut down on the expensive trips until he can afford to pay for it himself.