Why did Aquinas think an erroneous conscience binds?

*Quodlibet* III, q. 12 a. 2 co. : >it must be said that every conscience, whether right or erroneous, whether in things evil in themselves or in things indifferent, is obligatory, such that he who acts against conscience sins. > >Latin :
Et ideo dicendum est quod omnis conscientia, sive recta, sive erronea, sive in per se malis, sive in indifferentibus, est obligatoria; ita quod qui contra conscientiam facit, peccat. > >Davies, O.P. & Nevitt transl. :
conscience is always binding, whether it is mistaken or not, and whether it is a question of things evil in themselves or morally neutral. Therefore, it is a sin to act against one’s conscience.