They're most likely high-density modules, which are cheaper to manufacture. AMD CPUs, with their integrated memory controllers, do support high-density RAM; however, Intel chips of the era didn't have the memory controller on the CPU and the memory support was determined by the motherboard rather than the CPU. The problem is that Intel motherboards back then rarely supported high-density modules, so it's just easier for ebay sellers to market high-density modules as "AMD only" than to deal with angry, unwitting customers with Intel systems.
This has never been much of a problem with DDR3 - the memory controllers on Intel CPUs support high-density RAM so high-density DDR3 support on Intel system is pretty much a given.