See further comments below for an advanced analysis.
It should be recognized that for Black to obtain unconditional life, Black must start a ko at and win it. However, will take the ko first.
On the other hand, if White must kill the group, then must start a ko. In this case, Black will take the ko first at .
Hence, this ko is unfavourable for either player.
Of course, has the option of making seki. But Black lives this way.
Going further into advanced material, one should then see that the situation in this problem is actually some variant of a ten-thousand year ko, in which neither player wants to start the ko too early in the game.
However, these things are paradoxical. If White is komaster, Black should often start the ko, even though he loses it. The point is that he gets a play elsewhere while White wins the ko. If he waits, then the value of that play will probably diminish.
Things are more complicated when Black is komaster. If other plays are worth enough, then White should normally start the ko for the same reason that Black should when White is komaster. Then there is a temperature range where neither side will typically want to play, and then at lower temperatures each side will want to play because White now threatens to make seki.