Life In 19x19

this move was played by kobayashi koichi

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[go]$$c
$$ -------
$$ | . . . . . . .
$$ | . . O . . . .
$$ | . . O X X b .
$$ | . X X O O X .
$$ | . X O . . # .
$$ | . d . O a . .
$$ | . . . c . . .[/go]

i looked it up on kogo's joseki dictionary and they recommend A B or C as moves for white, clarifying that D is a game-over mistake because of the throw-in at D15
why is this not the case for B?

edit:
it even goes on to suggest this continuation

Click Here To Show Diagram Code

[go]$$Wc
$$ -------
$$ | . . . . . . .
$$ | . . O 4 . 2 .
$$ | . . O X X 1 3
$$ | . X X O O X .
$$ | . X O . . X .
$$ | . . . O . . .
$$ | . . . . . . .[/go]

i don't understand why black would ever play instead of D15

It is. Kogo's has mistakes. Josekipedia.com is better (but not perfect).

Yes, White has to defend.

According to Takao Shinji's dictionary, this is the equal for both variation: