What they actually say:
"Under no circumstances should you leave a laptop powered on and in any sleep/hibernate/standby mode when placed in a bag, backpack, or in an overhead bin. The PC will overheat as a result of that action. Any resulting damage will not be covered by the Dell warranty."
Fair enough. A laptop that overheats because it's been left in an enclosed space whilst not off is not damage caused by a fault of the device.
What they actually say: "Under no circumstances should you leave a laptop powered on and in any sleep/hibernate/standby mode when placed in a bag, backpack, or in an overhead bin. The PC will overheat as a result of that action. Any resulting damage will not be covered by the Dell warranty."
Fair enough. A laptop that overheats because it's been left in an enclosed space whilst not off is not damage caused by a fault of the device.