إِنَّ الَّذِينَ آمَنُوا وَعَمِلُوا الصَّالِحَاتِ كَانَتْ لَهُمْ جَنَّاتُ الْفِرْدَوْسِ نُزُلًا
Those who believe and do righteous deeds will have the Gardens of Paradise as hospitality.
A relative clause can suggest why a predicate is appropriate to its referent. In this verse, belief and righteous action provide a meaningful connection with the promise of Paradise.
This is rhetorical indication of a reason, not a claim that every relative clause is inherently causal. Many relative clauses simply distinguish who is meant.