One large issue is that you'll often not have a chance to learn from your mistakes and do better in the future. If you jump off of a building, you die. If you drink the big gulp for 40 years, you get diabetes. Sure, you may have learned from it, but you can't start over.
I'm still in the camp of "let them", but it's a mixture of "it's hard to figure out who really knows the risks", "I don't want to live in a world that's optimized for perfect safety and takes away all freedom to achieve it" and "we want people to take risks, even giant risks, even when they clearly have no idea how large the risk is, because we'll advance much quicker because of it, we just don't want all people to take those risks at the same time".
I'm still in the camp of "let them", but it's a mixture of "it's hard to figure out who really knows the risks", "I don't want to live in a world that's optimized for perfect safety and takes away all freedom to achieve it" and "we want people to take risks, even giant risks, even when they clearly have no idea how large the risk is, because we'll advance much quicker because of it, we just don't want all people to take those risks at the same time".