Crating a dog at night can seem cruel but some dogs need to be crated at night to prevent them from getting into things at night while you're asleep.
I used to have a dog that would wander around the house at night when I was asleep and then get into the cabinets etc and he got into some borax powder and almost died as a result.
So I started putting him in a crate at night to prevent him from getting into things and if he would just sleep with me in my room that would have been great but he won't.
I tried tying the cabinets closed but he chewed through the child proof lock on the cabinet and still got into the cabinet so in the crate he had to go.
Most dogs will eventually get used to being in the crate at night and feel a sense of security when you confine the dog to the crate.
In my opinion if you're crating or kenneling the dog at night or while away for good things then it's not cruel.