In-group vs out-group. States can take land from out-group, not from in-group (which is why the in-group participates in that state). Also, as someone else poin

In-group vs out-group. States can take land from out-group, not from in-group (which is why the in-group participates in that state). Also, as someone else pointed out, this land was taken from Mexico, who in turn took it from the Spanish crown. The Spanish took it from the "natives" (scare-quotes since I don't know if that tribe took it from another preceding tribe).