Here is a link to their podcast where Cassie first mentions getting on OF:
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Why they ended up getting on BrandArmy instead of OF remains a mystery, but I wouldn't be surprised to find out that the BA people reached out and offered them some sort of exclusive deal. Cassie mentions OF by name, makes no mention of BrandArmy, and she's the top related search query to BA. If you look at Google Trends comparing OF and BA, you'll see that BA isn't even on the radar when compared to OF (duh lol). She put herself at a significant disadvantage by choosing to get on BA instead of OF, so the only thing that would make sense is if they were offered extra incentive to choose BA.
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Also, just FYI we won't be seeing any nudes from them as long as they're on BA, as the BA TOS explicitly forbid nudity.