Probably an internal political thing
Perhaps it is a warning like you say. Something to tell them that if they make any moves towards relocating manufacturing out of China, they will find themselves shut out of the market. Since Foxconn is a Taiwanese company, China doesn't have direct control over them, so applying pressure on their customers is probably the best way to control them. But I doubt it.
This book/movie ban is symbolic, Apple's book/movie revenue in China is a rounding error on a rounding error, so it doesn't actually hurt Apple, it just makes headlines. There are so many other things they could have done if they actually wanted to make a real impact.
Rather than a warning shot, it might be a situation where they had to do "something" against Apple since they'd named them among the eight. So they chose something that wouldn't really hurt Apple and cause them to react, but would still make headlines that they're "getting tough" on US companies. Just like US politicians like to talk tough or take symbolic actions to shore up their political base or bring attention to themselves, a similar dynamic probably exists in China.
Biting the hand that feeds IT
