I'm gonna scream!

Nope its not cruel at all. Think of it more like them having their own private place to get away from the rest of the world. Give them a chew toy, make sure they had oppertunities to to clean out and do their business, and give them a safe secure place to hang out for awhile.
Never use it as a place to punish or "time-out" it should be a "good" place tp go. I leave my crates open while I am at home and my dogs use it as a place to get out of the way, they will put themselves up and take naps through out the day. I just make sure its all positive when training them to go in with a command "get in your box" the same command they get when we load up to go hunting,"get in your box" I can say it any time, anywhere in the house and they run to get in. In the beginingg they always got a treat, now only occasionally do they get a treat as a positive reinforcement to a good behavior.