Watching people like BHL and Ben Stein invoke the droit de seigneur, I can distill at least two:
If you report an assault soon after it occurred, it proves that you couldn’t be traumatized enough to have been assaulted. If you report an assault a few years after the fact, this proves that “you see an opportunity.” (Particular to this case: obviously, it’s completely irrational to think that if you report on being assaulted by a famous person his asshole buddies will immediately smear you with no evidence.)
If there is no history of the alleged attacker having committed previous assaults, the accuser could not possibly be telling the truth. If there are multiple accusers, this proves all of the women making accusations are lying, gold-digging sluts.