It's not quite that the Dunlendings were "descendants of the House of Haleth" so much as that they (and, generally, the population of Eriador from the Bree-land south) were part of a rather large race or tribe of Men of whom one branch crossed early into Beleriand and occupied Brethil. Similarly, the Northmen/Eotheod/Rohirrim were "cousins" of the Hadorians, but not descended from them.
On the other hand, maybe it was all because the Dunlendings were still ticked off over Turin and Hurin.....
