Sometimes the laughter is awkward, that of a confused person who really doesn't know how to handle the situation.
Sometimes the laughter is cruel because those around them or ignorant and don't understand so they laugh at their expense.
People with downs or any other disorder has a right to be in public. Those who find it shameful <i>sound</i> vain--concerned with appearances. Now, I'm not saying they are, but that's what shows. And you know, you cannot control how other people see you.
I have worked with down syndrome and handicap kids before. I would have stopped to stand in, and asked the caretaker if there's anything I could do to help. If she said "no" I don't know, but I think I would have walked away because there's no use in being no help and making a greater spectacle than there already is.
It also depends if I was feeling gutsy enough to say something. Sometimes, I just let things be, but if I had enough anger/concern, I might have said something along the lines of "It's not funny. Go away."
People like that really really "peeve" me off.
I understand though, that you had some where to be.