even sick jokes can be funny if they are cleverly constructed and make you think or something, people are not laughing because the subject matter is hilarious and not a serious issue, but at the joke itself ... a joke about someone dying in a bizarre silly way doesnt mean you think death or people dying is funny...
same as laughing at someone who trips over in the street - it may look funny, but actually it isnt really - they could be hurt... but we still laugh...
....but sometimes its just thinly veiled lazy racism, homophbia etc disguised as a joke.
it can be a fine line sometimes