Because that's what people do? Because they are angry? Because they are upset?
And if we need it, I'll clarify again, the fact that they will be angry is a natural, simple reaction to loss - no me saying the pensioner did anything wrong. Quite the opposite, he and his wife are clear victims and have had a horrible ordeal. And yes, they will also be upset that flowers appearing across from/near their house. But that in itself doesn't mean the burglar's family won't have the normal, understandable reaction to a violent death.