I agree they were thoughtless, but I think the issue might have been with regard to "If they knew they were going to the memorial, you would have thought they should have deflated the darn thing, at least."
The memorial is not far away from anything, you don't have to travel to get there, like you would a cemetery or concentration camp, it is right there in the middle amongst shops, high rises etc. and noise of the city. I was in NY last year and loads of building work was still going on around One World Trade Centre and you could walk past and not know you were at the site of a memorial (from some sides, literally couldn't see it for building work). I would think the memorial museum, which is underground, is far more solemn. I suspect that this was not a planned trip, but on the way to somewhere else.
I agree that they were thoughtless (and the doll is just sad/pathetic/try hard, regardless), but, sadly, it seems selfies appear to be order of the day and the site for a lot of people is a tourist attraction rather than a site of memorial/reflection.