First of all, let me correct myself. Anand did not offer a draw. He took Magnus' last pawn. Magnus (White) had a lone King; Anand had a lone King and a Knight. This is insufficient for either side to force checkmate, and therefore a draw by the rules.
In any case, it would have been considered poor form for Magnus to make the offer. Anand knows very well how to analyze a position and knew he couldn't win. That's different than football, when you may as well run a desperate play because it just might work. In addition, Magnus is the highest rated player ever. It would have been just rude for Anand, in effect, to say, "I think you'll blunder this drawn position away."
In the Tal Memorial Tournament in Moscow earlier this year, Magnus turned down a draw offer in the last round that would have given him the tournament championship and a considerable amount of prize money on a silver platter had he accepted. The draw was offered by Levon Aronian, who would have tied Magnus for first place had he won the game. Magnus turned down the offer on principle; he simply didn't think the game was definitely drawn. He went on to win the game.
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