I would imagine that the anonymous poster either doesn't share the opinion that their behavior is inappropriate OR is of the opinion that "digging up the past" is totally worth the negative judgement.
Although, I'm beginning to agree with the saying "trying to understand people is like trying to smell the number 9".
What is it specifically that bothers you about their bringing up the past?
Try this school of thought. The deceased is not reading, they are not offended. Rather family and loved ones, who are well aware of the deceased wrongs find it offensive in their time of mourning. You may not like what a family member has done, but for a parent, that person is still your child and the loss hurts. You don't really want to have their past dug up and smeared across the internet.
What I think the OP is referring to is the propensity for some to read an obit and then use that to berate the dead.
If you all don't have a least one black sheep in your family, you are either a nun or an alien.