x86-64 ISA oddity

New processors have FXSAVE and FXRSTOR to save/restore the floating point state of both x87 and SSE units. In 64-bit mode, PUSHAD and POPAD instructions generate undefined operation exception, there is no new instruction to save/restore all general-purpose registers, and the hardware task switching is disabled. I'm wondering why did the AMD designers decide to cripple the CPU in such a way (along the side of disabling segmentation, but that's a story for another post).