I have a booking on UA CLE-DEN-RKS flying first class CLE-DEN and a connecting flight in economy DEN-RKS. I'm in economy on the DEN-RKS only because it's a single cabin flight. I can select any seat on the CLE-DEN leg but can only select seats in rows 7 and back on the DEN-RKS flight, with those forward of that incurring an additional charge. I don't understand why when I'm flying in first on the longest segment that I can't select a better seat on the short segment.