I posted a video on another forum that allows such things because if I posted it here half of you would die of shock and I would be banned forthwith but anyway the guy says that the reason humor is funny is because it is usually looking at dark subjects involving the human condition. Most jokes are poking fun at our own human failures and being able to laugh at it is our way of dealing with it by transcending the dark for just a moment and allowing ourselves to process the sad subject in a moment of light.
When a loved one dies, and the grieving family comes together, there are funny stories told about the deceased. It is a relief valve from the intense emotion. Some stories or jokes can be quite dark. When a friend of mine committed suicide his other friend told a joke about the method he chose. It was so funny we laughed until we cried (literally because we're also crying with grief) but the reason it was funny was because it perfectly summed up our friend's life and personality. And it momentarily allowed us to transcend the horror of what he had done by viewing it as the logical outcome of his own character.
But here's the thing: We would never tell that joke to strangers or anyone who was not in our inner circle. Such humor is only appropriate in certain private contexts. So no, I would not buy that t-shirt. Not being close to that guy, I don't "get it" if it's meant as transcendent humor about him, and if it's meant to honor what he did, I completely disapprove because he stole property, put other lives at risk and probably will cause costly changes in security theater.