I imagine the folks at Wikipedia had a few incidents of people being helped with their depression and killing themselves subsequently (possibly as a result of the help they received), then Wikipedia was like "What should we do? How about a don't-talk-with-suicidal-people policy?" without looking at the other, much more obscure cases where someone asked for help and did not subsequently commit suicide (possibly as a result of the help they received).
I don't know. Wikipedia spends a lot less time on socializing then LW does.
ETA: gwern, what's your take on http://www.reddit.com/r/suicidewatch?
No one can stop them even if it's a terrible idea, they have nothing to lose, and nevertheless, they list their list of suicide hotlines prominently in the sidebar.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.