A lot of the RISC architectures do something similar (sign extend rather than zero extend) when using 32 ops on a 64 bit processor. MIPS and PowerPC come to mind off of the top of my head. Being careful about that in the spec basically lets them treat 32-bit mode on a 64-bit processor as just 'mask off the top bits on any memory access'. Some of these processors will even let you use 64bit ops in 32bit mode, and really only just truncate memory addresses.

So the real question is why does x86 zero extend rather than sign extend in these cases, and the answer is probably that by zero extending, with an implementation that treats a 64bit architectural register as a pair 32bit renamed physical registers, you can statically set the architectural upper register back on the free pool by marking it as zero rather than the sign extended result of an op.