The infallibility of the Prophets (a) and ‘Aimmah’ (a) is considered one of the important beliefs of the Shia. For a deeper discussion of this matter you can refer to the questions: 1737 (site: 1733), 1739 (site: 1734), and 3026 (site: ) in this same website.  In reference to the question which you asked it has to be said that in the original Arabic text of ‘Aqaidul Imamiya’, written by Allamah Muhamad Reza Mudhaffar who is counted as one of the famous and great Shia scholars, we did not find any reference to such a statement which said that the infallibles are forced not to sin and that they have no choice in the matter. In fact, in certain traditions, the opposite has been mentioned that they do have free will. Even more explicitly we have the words[1] (of Allamah Mudhaffar) who said in regards to the infallibility of the ‘Aimmah’: ومن الامور التی یجب التنبیه علیها ایضاً هو ان المعصوم(ع) لیس مجبوراً علی فعل الطاعات و ترک المعاصی، و انما فعل و ترک باختیاره؛ لانّ العصمة ملکة و بصیرة فی الامور؛ اذ انه ینفر من المعاصی و یبتعد عنها، و یتشوّق الی الطاعة فیعملها، و کل ذلک بتوفیق من الله عز و جل[2] which means: “One matter which is important to mention is that the Infallibles (a) are not forced to obey God and not commit sins; they act based on their own free will. This is because infallibility is a type of insight and inner vision into matters and the person who is infallible has an aversion towards sinning and stays far away from them. Furthermore, they have an eagerness towards obeying God and act based on these principles. While these things are not forcing them into predestination, they are still a great favor and grace from God.

Hence, the words of the late Mudhaffar clearly point to the free will of the infallibles and not to any sort of predestination. It is possible that in the English translation of the text there was a misprint or mistake. It is also possible that in the reading of the text a misunderstanding occurred.