Urban Outfitters is Obsessed with Navajos
First of all, these products represent a stereotype of "southwest" Native cultures. The designs are loosely based on Navajo rug designs (maybe?) or Pendleton designs, but aren't representations that are chosen by the tribe or truly representative of Navajo culture. Associating a sovereign Nation of hundreds of thousands of people witl a flask or women's underwear isn't exactly honoring.
Additionally, it's more than likely that Urban chose "Navajo" for the international recognition--to most of the world Navajo (and Cherokee)= American Indian (my Jamaican friend didn't even know there were other tribes in the US until she met me). This conflation of Navajo with "generic Indian" contributes to the further erasure of the distinct tribes and cultures in the US and solidifies the idea that there is only one "Native" culture, represented by plains feathers and southwest designs.
For more on fashion appropriations, see "Native Wolves" Spirit Hoods and "Cherokee Chic" = Generic Hippie Look.
Below: Navajo Hipster Panty.