Originally posted by crythias
I think that's because the board becomes "Dumb" in analyze mode. the only way to do en passant is to attack the opposing pawn, then move the pawn forward. It doesn't castle automatically, either :-).
Yes, only the server understands the rules of chess; that's why, when you castle on the game page, you move the king and then, when you press 'Move', the rook moves when the screen refreshes.
To say that 'Analyze board' is dumb is a little unfair - it's way beyond what I could imagine doing in a client-side script - but you can't expect it to know much more about the rules of the game than a wooden chess set does.