Is Crate Training Your Dog Cruel?

No it is actually very useful for housebreaking, traveling and destructive or anxiety behavior. 

That being said leaving your pet in a crate for less then 4 hours or if that's where they sleep at night is not cruel.

 Now leaving your pet in crate 8+ hours or longer without any supervision or getting checking on can be thought as cruel.

 Leaving a pet in a crate 24/7 is outright animal abuse and should be avoided at all cost no matter what the circumstance.