Answer

The logic is that his eating is an act of commandment-fulfillment. Especially if he intends it as a commandment, then what difference does it make that there was duress here? We are back to the question of double intention. Think of a situation where you are eating matzah, and meanwhile someone is holding a gun behind you and planning to shoot you if you do not eat. Have you not fulfilled your obligation? So if he is threatening you verbally, and still what you are doing is for the sake of the commandment, why should you not fulfill it? It is absurd that an act that constitutes an act of commandment-fulfillment, and through which you fulfill your obligation, should lose its significance just because someone decided to threaten you. However, if you would not have eaten without the threat, perhaps you did not fulfill it (and one could also discuss this as in the case of “they coerce him until he says, ‘I want to,’” where the coercion generates genuine will. That also seems to be the meaning in the Talmudic passage in tractate Rosh Hashanah, “the Persians forced him to eat matzah,” where the discussion is about a case in which he would not have eaten without the coercion). See also the responsum regarding The prohibition of revenge when there are additional motives.