Those aren't stars, those are hot pixels and "shot" noise. The reason you see more against the backdrop of space than against the Earth is because the Earth is so bright it drowns out most of the noise in that part of the picture.
The image was shot at ISO 400, f/16, 1/640 seconds. That's basically the same exposure settings to use on a white sand beach or a snow covered ski slope in direct bright sunlight! It's slightly less exposure than the so-called "sunny 16" rule-of-thumb which says that when your subject is in direct mod-day sun at f/16 you should use a shutter time that is the reciprocal of the film speed (film) or ISO setting (digital).
Stars aren't bright enough to break through the noise floor at such exposures, even without the Earth's atmosphere in the way. The colored dots you see are image noise.
Image noise has two main contributors:
- Read noise. This is electrical noise caused by the camera's operation. Though modern cameras have become very good at suppressing most read noise, one would need to cool the sensor down to absolute zero (0K or -273°C) to theoretically totally eliminate all read noise.
- Image noise caused by the random nature of light itself will vary from frame to frame. This is often called "shot" noise or "Poisson distribution" noise after French mathematician Siméon Denis Poisson, who developed a discrete probability model that mathematically expresses the phenomenon observed in nature. How much it will affect an image will depend upon the strength of the light falling upon the sensor, how long that light is allowed to fall on the sensor, and the size and quantum efficiency of the sensor's discrete photodetectors a/k/a pixel wells, photosites, or sensels.
In the case of the Nikon D4 aboard the ISS that took this shot, there appears to be a little more read noise than would normally be expected for that camera. The D4 was introduced in 2012 and discontinued in 2014 when it was replaced as Nikon's "flagship" stills camera by the D4S in early 2014. We can guess that the camera has been aboard the ISS the better part of a decade. Outside of the protection of Earth's atmosphere it is exposed to higher levels of radiation, particularly gamma rays, than a terrestrial based camera would be. Gamma rays are known to be a cause of image sensor degradation that results in increasing numbers of noisy sensels with cumulative exposure to gamma rays and other types of short wavelength/high frequency EMR.
The Reason You Don't See Stars
If you expose bright enough to see even the brightest stars in the sky, even the dimmer Moon, which is a little more than one stop dimmer than the "Sunny 16" rule (We sometimes call it the "Looney 11") will be completely blown out. The only two differences in the following two photos that contain the Moon and much dimmer planet Jupiter is the amount of exposure and that one image is rotated about ninety degrees compared to the other.
To get even the very brightest stars (and four of Jupiter's Moon that were also within the field of view in the first image) to show up, the moon is so overexposed that light from it is bouncing around inside the lens a reflecting off the front of the sensor stack and the front of some lens elements and then off the back of other lens elements. These same reflections were also bouncing around in the lens in the first photo, but they are so dim as to not be visible at all because the first photo was exposed 12 stops dimmer than the second image, or with 1/4,096 as much light allowed into the camera.
Also note that the exposure was long enough that the stars trailed past the camera mounted to a stationary tripod.
For a couple of intermediate exposure levels between the two extremes above which illustrate that details of Jupiter's bands and Jupiter's moons can not both be properly exposed at the same time, please see this answer to a different question.