I know older people who told me they could hear the screams from the stadium, just don't understand how people could find that funny.
Because jokes like that don't mock individual victims, and there's a sense of detachment from something like that. There's a huge difference between laughing at individual victims and their families from 9/11 and "Don't make 9/11 jokes, they're just plane wrong". I can promise that not a single person would go meet a family that was affected by it and laugh if they told their experience. But I think if you were to really sit down and ponder every terrible thing that happens in this world, you'd die of depression.
Every offensive joke has the ability to mock people's pain, that's why they're offensive. Rape jokes, dead baby jokes, jokes about the holocaust, sexist jokes, religious jokes, morbid jokes, etc... That's exactly my point about it being just a joke. Whether you find it funny or not isn't the point.
Like all jokes, it's about context and knowing your audience. If you know people from the Hillsborough disaster, don't tell jokes about it around them. Or make sure your friends have the same humor as you before telling a joke. I don't tell racist jokes to my dark friends unless I know they're cool with it and understand it's a joke. That's not so hard is it?
But it's a little ridiculous to say that people shouldn't be allowed to laugh at certain things just because you feel uncomfortable with it. I have family in NYC, my uncle only avoided getting killed in 9/11 because he woke up late and decided to just work from home instead, his entire company was taken out an hour after he got off the phone with the boss because a plane crashed through those floors. My cousin watched people jumping out of the buildings. I still remember the panic of frantically calling family to make sure they were all ok. I still laugh at 9/11 jokes and I don't hold it against people that do either. Nor do I scoff at people that don't find them funny, and I would never tell those jokes to people that were uncomfortable with it. It's not about mocking victims and their pain, sometimes its about finding a way to detach yourself from tragedies like that and sometimes it's just about funny and tasteless shock value.
But laughing and not laughing at a joke like that doesn't really say much about you as a person, and it doesn't make you a better person to proclaim that certain jokes just aren't funny.