I meant that retirement or withdrawal doesn't lead to the use of the BU in every situations
It’s my understanding that the backup is always used if either player withdraws, even when an LL is in play. For example:
DAY01 has Bertens vs Lisicki, and you pick Bertens. If lisicki withdraws, a LL is put in place, Mladenovic. Even though Bertens still plays, your pick is invalid and a back-up is used. This is because you picked Bertens to beat Lisicki; she now plays Mladenovic, making your pick invalid.
At least, that’s how I’ve always understood the rule
now that you ask about it though, I could be wrong
In which case, I’d argue that a “walkover” is different than a withdrawal before the match starts.
More importantly, back to your initial question of WHY this rule was in place for Osaka. The biggest reason is that we never know WHEN a player withdraws. Sometimes it’s done in press conference, sometimes on twitter, sometimes not until a player walks on court. So, in theory, often it’s nearly impossible to tell WHEN the walkover was actually granted. Therefore, I could pick Bertens after I knew Osaka had a walkover, but before it was made official, thereby making it impossible for the tournament runner to know whether my pick was made on time or not. Essentially, “start time” is impossible to determine, unlike a normal match where start = “first point”
This, plus the other obvious reason of the fact that no match was actually played, are the two big reasons why we void a walkover no matter what