Because they went public about it, he had Twitter deactivate the account of one of them, the other has been intimidated by his followers until she has made her account private, he has filed a $20 million lawsuit based on his lies, and he has, with nobody to speak for them in court, successfully requested that a judge grant the ability to subpoena Twitter for the identities of his accusers.
It's business as usual.
From Google for "Justin Bieber":
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