I don't get why [rock] bands never smile in photos.
Or when they scowl as if they hate life or even flip the middle fingers.
It just looks so fake and forced and lame.
Depends on what fits the band. A lot of band photos would just look stupid if they were grinning. A promo image should fit the image of the band, there's no one choice that fits every band from every genre.
In the case of DT, it does look forced, because we know they're light hearted fun guys. I'd love to see a promo photo where they're looking happy and informal, because they don't suit the neutral look. In the case of many other bands, a photo like that would be perfectly fine.