It is not cruel and it is not terribly hard to catch fox (or anything for that matter) in leg hold traps. My dad traps every year and I love to go with him. I have NEVER seen a fox even nibble at it's foot and I have NEVER seen a fox with a broken foot in one of these traps. My dad uses offset traps, but I know plenty of other people that don't use offsets and nothing happens to the feet.
My dad has caught pet dogs and cats that have been allowed to roam (dad has requests from sheep farmers to trap nuisance animals, sometimes these animals are pets) BUT they have always been let free with not even a scratch.
I have seen several animals asleep in the trap when we walked up..not often because I talk a lot..but often enough. Every trapper I know has gotten a hand/finger caught in a trap and they have all 10 working digits.
They are called leg hold traps...not leg-catch-break-maim-torture traps.