Suppose an experienced teacher, before the final examinations, predicts that a particular student will fail in the exams, since the student is very mischievous, not attentive in class and does not do his homework. If after the student appears for the examination, he fails, who is to be blamed for the student failing: the teacher or the student? Just because the teacher predicted, it does not mean that the teacher is to be blamed but the student himself is responsible for his failure.
Similarly Allah (swt) knows in advance that there are some people who are bent on rejecting the faith and Allah has put a seal on their hearts. Thus these non-Muslims themselves are responsible for rejecting the faith and not Allah (swt).
