The understandable consumer desire for thin lightweight devices with high performance components and the minimum of fan noise makes for a difficult set of design trade-offs.
Even with the use of 15W Thermal Design Power 'U' series processors, these devices can generate an great deal of heat. The thin lightweight form factor limits the total volume and mass available for the heatpipe(s) and heatsink, whilst the small fan(s) can only move a limited amount of air. To limit the amount of thermal throttling, the processor is allowed to run pretty hot - up to perhaps 85°C. All this means that the areas of the system around the thermal solution as well the exhaust air get rather hot.
I doubt these systems were overheating - they are designed to throttle down their performance to keep within safe limits for the hardware when they are running hot, also they will switch off automatically if thermal limits are breached. However, they are probably not suitable for use on the lap unless some sort of pad is used under them or you set a power plan that limits the performance to reduce heat generation. These issues are not unique to HP; I have experienced them with similar products from competing manufacturers.
For anyone interested in monitoring the internal temperatures of various components together with other parameters, I recommend the free HWiNFO. If you are after a pad to use your computer on your lap, one possibility amongst many is Belkin's Laptop Cushdesk.