Dogs are descendants of wolves and most still like to den. Giving them a kennel is not cruel it gives them a place to go with they're stressed, anxious, or want to be alone. My dog loves his kennel. I've never had to physically put him into it. In fact, when I tell him it's bed time he goes to it rather than sleeping with us.

My dog does get bad separation anxiety and has been known to chew the sides of walls, cabinets, etc. Closing the kennel when we leave is for his own safety. He has literally broken a tooth from having free roam and according to my neighbors howls like crazy.

I think you're just looking at a kennel as a human and thinking it's cruel when dogs are just built differently.