Why does iPhone capture screenshots as PNG? Is there a reason for iPhone to save screenshots taken as .PNG instead of .JPG?

Screenshots won't have transparent background anyways.
PNG is not only useful for allowing transparency: PNG is a lossless format, while JPEG uses a lossy form of compression. Also PNG does compress screenshots pretty well (unlike camera photos, which is why Apple uses JPG for those).



More info if interested:



Also, A technical overview of what PNG is good for is worth reading (it's basically one of the main references on the topic and contains enough details to spend the day on it). Excerpt:


  For the Web, as of early 1999, there are two image formats with
  ubiquitous support: JPEG and GIF. JPEG is very well suited to the task
  for which it was designed--namely, the storage, transmission, and
  display of photorealistic 8-bit grayscale and 24-bit truecolor images
  with good quality and excellent compression--and PNG was never
  intended to compete with JPEG on its own terms. But PNG, like GIF, is
  more appropriate than JPEG for images with few colors or with lots of
  sharp edges, such as cartoons or bitmapped text. PNG also provides
  direct support for gamma correction (loosely speaking, the
  cross-platform control of image ``brightness'') and transparency. I'll
  discuss these in more detail shortly.
  
  GIF was the original cross-platform image format for the Web, and it
  is still a good choice in many respects. But PNG was specifically
  designed to replace GIF, and it has three main advantages over the
  older format: alpha channels (variable transparency), gamma
  correction, and two-dimensional interlacing (a method of displaying
  images at progressively higher levels of detail). PNG also compresses
  better than GIF in almost every case, but the difference is generally
  only around 5% to 25%, which is (usually) not a large enough factor to
  encourage one to switch on that basis alone. One GIF feature that PNG
  does not try to reproduce is multiple-image support, especially
  animations; PNG was and is intended to be a single-image format only.
  A very PNG-like extension format called MNG has been developed to
  address this limitation.