A) WD (by a government) or its staff (by WD management) were ordered to put in the backdoor, but didn't agree with doing so, thus made it obvious in the hopes that it would be found.
B) A backdoor that is found but written off as sloppy development is less damaging than a bug that if found and analysed looks deliberate (because bad development practices are hardly new for hardware manufacturers). _Potentially_ makes exploiting it less risky as well - if it's an obvious or known thing, the attacker could be anyone. If it's a subtle, undisclosed bug (that hasn't been used against many targets), that suggests, to some extent, the involvement of whomever could arrange for the bug to be placed there.
It probably isn't deliberate, but that possibility certainly isn't excluded either, so I'd be cautious about treating this as a hard and fast rule.