Ah, good point!
..that actually made me think of Locke for some reason..
I'm also wondering how not being able to die, but being able to suffer physically (and mentally) relates to Alpert, Mikhail and the likes of Locke who the island appears to favour atm. Maybe the island favours Locke partly because it knows what he has gone through - it knows that he has survived great falls yet survived? Perhaps the island needs someone who is a survivor, someone who is death-proof even without any intervention or 'cushion'?
In saying that we could very well have the situation where the island itself prevented Locke from dying, not just when he was in the pit and not just after the 'crash', but all those years ago before he even came to the island..
I guess the knowledge that the island can indeed operate at an off-island level makes this possibility more possible..
I mean even Locke himself believes that the island brought him there, so i guess it also means that either the island brought him there because of his natural ability to 'survive', or because the island itself made him survive that fall and planned most things that have happened to him along the way..