If the Vedas be considered authoritative everywhere, then the Smritis also would be considered authoritative, for the latter are based on the former. But when the Shrutis and the Smritis contradict each other, how can either be authoritative.
ते चेत् सर्वप्रमाणं वै प्रमाणं ह्यत्र विद्यते। प्रमाणेऽप्यप्रमाणेन विरुद्ध शास्त्रता कुतः॥