In the case of a digital image, the only important parameter that determines the resolution is the width and height in pixels. The DPI parameter simply tells the software which size IN INCHES (or cm or mm) should be assigned to the image.
In fact, your camera does not take a 300 DPI image, because the size of the output medium is not known.
When RT saves an image at 72 DPI it does not reduce the resolution, but only changes the assigned METRIC dimensions. This can be changed later on without changing the actual pixels.