That's a Compilation thing...and even then not a consistent one. I was fine with Kadaj doing it because he wasn't really corporeal to begin with, but I don't know why normal people would.
I've always wondered this: why didn't Cloud bury her body in that water, near the podium thingy she died on (where the white materia fell)? He carried her up all those glowy stairs and out to that little lake (where Kadaj stood with the kids and drank the water).
Which would make you wonder how Cloud was able to carry her body that far in order to give her that send off and also how they would eat meat as all the critters would disappear before they could butcher them as a foodsource.
Have you all seen GameTheorists explanation of this? Its hilarious and points out that Aerith may not have been dead when Cloud "buried" her.