Comedy equals tragedy plus time...100 years is not equal to 13 years.
I don't really find those other examples funny anyway, but at least I find them less offensive. Though as a rule I find Hitler humor to be worse than natural tragedy humor, if I have to put it on a scale.
I *really* hate the "how is this fine if that isn't" argument. As if one is okay because the other is, when you've made the assumption I find the other okay when I haven't even heard of it.