If you're opponent uses Torment on your Pokemon that just so happens to be Banded (or choiced in general) then what move do you do next? Is it struggle? but that's a different move.
What happens next???
As you Said it'll be struggle because of you can only use 1 move but it't tormented. Disable would be better though.
If your Pokemon is choiced and Torment is used on it, it would go into Struggle. The opponent can't use Torment again until the last one wears off.