This is because we will "fallback" to using the OP_INSERT/OP_UPDATE opcodes which don't support this feature.

Had we used the normal write commands, then w=0 would be valid value and valid action.

The decision to error, rather then "upgrade to use write commands and w=0" is due to the fact write commands w=0 latency is dramatically higher than OP_INSERT – and the assumption is that would hurt users more.