Why didn't Fortunato resist being fettered?

1.  He was drunk and therefore inhibited.

2.  He was sick from the nitre, suffering from a dizzying cough.

3.  He was obsessed with finding the cask of amontillado.

4.  He was so naive as to Montressor's true villainous intentions that he found being fettered an absurd joke.

5.  He had no concept of revenge.  He kept repeating, "For the love of God," which shows that he expected Montressor to be a New Testament kind of Christian who, like Jesus, condemned revenge; instead, Montressor was an Old Testament believer in vengeance, like its God "Yahweh."