It deters minors. Your average English speaking parents are incapable of taking accountability when their child manages to get ahold of pornographic games. Since Android devices are much more accessible than PC devices and are often personal as opposed to shared like PCs, it is likely said minor would get ahold of the game if it were not for the price tag. By gating it behind a paywall, they're providing themselves with some level of legal protection. Wechat isn't used because it isn't a standard payment processor.