The gemara in Beitza 16b brings an episode about a blind man who recited tannaic texts in front of Shmuel. One yom tov Shmuel saw the blind man was depressed and asked him why is he down? The blind man responded that he forgot to make an eruv tavshilin and now cant cook for shabbas. Shmuel said you can rely on my eruv tavshilin like my fathers psak on this matter. The next year the same thing happened and the blind man forgot and Shmuel said that now you are negligent and you cannot rely on my eruv tavshilin.
My question is aren't the blind exempt from mitzvos (Rabbi Yehuda Bava Kamma 87a), and since the whole reason for eruv tavshilin according to Shmuel (daf 15) is not to forget food shabbas (one may eat all the food before shabbas) then why didn't Shmuel allow a blind man to rely on it if this mitzvah (safeguard of a positive mitzvah) doesn't apply?