The severity of the injuries is important because McDonalds had ignored many previous accidents and had ignored a CDC request to reduce the temperature.
Reducing the temperature reduces the severity of the injuries.
Using your marshmallow analogy: if you sold the marshmallows in individual plastic shells and children started eating the marshmallows by sucking them out of the shells, and children were sometimes choking because they were inhalling the confection, and you were told about this but ignored it, then yes you probably should bear some responsibility.
I case you think this is unlikely: http://www.food.gov.uk/news-updates/news/2011/mar/jellycupba...