An empty-looking place is not necessarily an invitation to insert oneself between two seated people. A brief request for permission may prevent interruption or discomfort. If they do not agree, choosing another seat respects their boundaries.
Example: Two people are discussing an important matter. A third person asks, “May I sit here?” and sits only if both agree. The phrase بِإِذْنِهِمَا makes their permission central. Courtesy is not just the choice of polite words; it also involves recognizing appropriate personal boundaries.