It’s also not a legal requirement of the claims (it’s just facts that play well to a jury). The threshold legal issue is did salesforce know or should SF have known their goods/products were being used by backpage to facilitate human trafficking/prostitution.
As for the subject matter if saleforce knew in 2016 what the FBI acted on in 2018, I don't know. It leaves me with several large questions, the biggest being about why the site lasted for as long as it did. The FBI received continuously information about child sex trafficking and prostitution, and had several court cases dating back to at least 2010.
From a moral perspective its an valid discussion to ask if saleforce should consider their ethical consideration of how their customers utilize their goods and products, but from a legal perspective I kind of look at https://en.wikipedia.org/wiki/Backpage#Legal_decisions and see a complete failure from the legal system to tell the world if the behavior of backpage was illegal. It would not surprise me if saleforced did ask their lawyers and got a OK back.