toragaki What the one guy was saying in the post I linked is that it's a marketing strategy to make us want more. Him and a bunch of other people started talking about how Yua Mikami's debut was also fake. Basically, when a certain girl goes from being an ex-idol (singer, gravure model, or movie actress) to AV, all of the girl's fans will be extremely anxious and highly anticipating their debut AV. This means when they find out that their debut AV isn't really a debut because of the fake penetration, they'll go crazy waiting to see the cute ex-idol get fucked for real. It builds a lot of anticipation, which boosts sales. It's a smart way to take advantage of ex-idols because everyone wants to see a girl who quit or failed outside of AV to get fucked.
A good example is all the people that want a lot of gravure idols like Ai Shinozaki to do a nude set or get into AV. They love the idea of an innocent girl like Ai Shinozaki who's teased them for so long to finally reveal her body or get fucked. It fuels a man's excitement as much as or maybe even more than a popular AV star finally doing anal.