>>1 I'm happen in the area of something like this as well, so these thoughts are currently in my head ;)
People generally don't speak openly when defaming/insulting a person, and in my belief it shouldn't be done mindlessly/without cause. However, it is frequently done 'behind backs' (which in my opinion is no lesser of an evil). But there is some reason to it, and it is that to speak openly is a liability to one's character. When someone dies though, it is a sensitive time for both family and public in general. For someone to speak ill of person openly or in confidence, that would be less likely to be taken lightly. i.e., it is more of a risk to one's own character. However, speaking positively has 2 major purposes.
1) For one who partook in the slander of the deceased person, it is tool to diffuse any possible bad feelings/blame toward themselves accrued by their slander. This is more commonly the case for those who respond with lines such as 'I miss (him/her) so much!', 'He/She was such a good person', etc.
2) It is secondly used as a form of repentance, or an escape from inward guilt (rather than a diversion meant for others).
Both these tactics are (to my notice) frequently displayed by passive bystanders. That accounts for the seemingly disproportionate amount of sympathy. The likely many people who passively observed the deceased being verbally battered are experiencing guilt for their unwillingness to remedy the situation, and are accordingly 'repenting' in what is the most easy manner available to them.
My point in saying all this, is that I believe this is the cause of what you're(we're) observing and it's something that's quite natural; and though irritating, I see it as rather unavoidable given the state of society and perhaps something deeper with us.