Ideally most of the various people in possession of part of the key or the encrypted data are secret (actually, more ideally, the encrypted data is publicly available). One anonymous tweet over Tor could spill the secret with relatively little risk to the individual.
There is a certain safety in numbers too, if you are one of dozens of people in possession of the key, if you have a little backbone you can leak the key and be reasonably confident that they are not going to kill several dozen people purely out of a sense of revenge.
This is why Glenn is so keen on pointing out that he is not the only person with the data/key.
The fact that others have the same info means that it makes a lot of sense.
Not a lot of people have the backbone to carry on after someone has been made an example. And those that do, well, they can also be made examples.