It's always tough to look at things from an adult perspective, isn't it?? Add to that the things we learn about the environment. For example, no one throws rice at weddings anymore - it's either bird seed or flower petals.
Water balloons are fun but we've learned that the shredded leftovers are so hazardous to birds and animals. I think marshmallows are similar - hazardous to wildlife and you just KNOW the kids aren't going to clean up. In some ways it's a shame the letter carrier couldn't look the other way, but then again, she might not have known at first what they were throwing. And after all those kids had choking problems a few years ago by having contests to see who could put the most marshmallows in their mouths, we learned that marshmallows melt with body heat and block the airway.
You're right that we live in a society where people sue at the drop of a hat, and that's a problem. You're also right that things do escalate, and while marshmallows might be completely harmless, kids who escalate to dropping rocks off overpasses which smash windshields, I guess we can't allow pranks to go unchecked.