It's mostly because the dead cannot defend themselves. You are attacking someone who you have no fear of reprisal from.

This has been mentioned a few times in this thread. But it doesn't really make a lot of sense, especially in the case of someone famous.

If two or three days ago, not knowing he was sick (which I didn't), I had said to someone "That Dilbert guy seems to be sort of a whack job," why would it matter that he was alive to hypothetically defend himself? It's extremely unlikely that he would ever be aware of my comment at all. So why does it matter that he's alive?

I didn't fear reprisal from Scott Adams when he was alive, either.

And there are plenty of people willing to step in for Scott and defend him, as evidenced by the contents here.

Someone dying doesn't mean the consequences of their words and actions disappear and acting like we should pretend that death washes away those consequences is silly.