It can work but it's a really, really painful and slow way to die. And this kind of prevention can work: a lot of suicide prevention is focused on just putting enough barriers in place that it's harder to do on impulse. In the case of the limitation it's probably mostly so that people don't normally have a huge jar of them at home, which is what you get with buying them from costco. How many lives have actually been saved by this is pretty hard to judge though, but I would bet it's non-zero.