How are oranges in the US or anywhere made seedless?
Oranges and other fruits are generally not actively made seedless. Rather, seeds may fail to develop due to either lack of fertilization (pollination) or a natural tendency. The natural production of unfertilized and thus seedless fruit is called Parthenocarpy.

To quote the Scientific American article (3) mentioned by Oreotrephes:


  Fruit development normally begins when one or more egg cells in the
  ovular compartment of the flower are fertilized by sperm nuclei from
  pollen. In some plants, however, fruit develops without fertilization,
  a phenomenon known as parthenocarpy. Parthenocarpic fruit has
  advantages over seeded fruit: longer shelf life and greater consumer
  appeal.
  
  The most frequent reasons for lack of seed development are pollination
  failure, or nonfunctional eggs or sperm. In many plants,
  self-incompatibility genes limit successful fertilization to
  cross-pollination between genetically different male and female
  parents. This property is exploited by citrus farmers who grow
  seedless fruits, such as navel oranges and clementines. Because these
  cultivars are self-incompatible, they fail to set seed when they are
  planted in orchards of identical plants (clones). These plants have a
  high frequency of parthenocarpy, however, so they still produce fruit.


Parthenocarpic varieties may arise from a lack of pollinators. From the Wikipedia page on Parthenocarpy:


  Plants moved from one area of the world to another may not always be
  accompanied by their pollinating partner and the lack of pollinators
  has spurred human cultivation of parthenocarpic varieties. Some
  parthenocarpic varieties have been developed as genetically modified
  organisms.


To preserve the seedless trait, parthenocarpic trees can be propagated by grafting. It is possible that more kinds of seedless fruits will be engineered in the future (3):


  Plant biologists have learned that if the plant hormone auxin is
  produced early in ovule development, parthenocarpic fruit can grow on
  plants that do not usually exhibit this property. Thus, genetic
  engineering will most likely give consumers parthenocarpic fruit in
  many other species in the near future.