I doubt they take actual screen pictures on their promo materials. This is usually something done in post-production, where you integrate the screen on the frame of the device. Or you make a 3D render altogether.
Actually, I recall a short documentary about the production of the Apple ads and as far as I remember Apple uses real photos of products, not CGI. But probably because of the difficulties of capturing photos of LCD screen(white balance, polarization, luminescence intensity and so on) they may be replacing screens with screenshots.
