What is the clear difference? levels of what is offensive are subjective. Wearing a swastika for example is deemed offensive, yet wearing a Viking hat isn't. Granted, they aren't wholly the same, but both sets of people committed horrendous crimes.
What these guys did was very immature, disgusting and offensive, but it wasn't and shouldnt be a crime. Where do we draw the line? we place an effigy of a man on a bonfire every year... is that not offensive?