Is it possible to see what was 'illegal' about my app? I've recently had an app of mine accepted for distribution on the iOS App Store. As usual, I selected to allow for publication on all App Stores. The app was approved and the app was available to download on all the international app stores.
Approximately 24 hours later, I received the following message in the Resolution Center:

We are writing to notify you that your application, application name, will be removed from the China App Store because it includes content that is illegal in China, which is not in compliance with the App Store Review Guidelines :
22.1 Apps must comply with all legal requirements in any location where they are made available to users. It is the developer's obligation to understand and conform to all local laws
While your app has been removed from the China App Store, it is still available in the App Stores for the other territories you selected in iTunes Connect.

Is it possible to see what it was about the app that was regarded as illegal?
I'm not aware of any content of my app that would be regarded as illegal, especially as my app was only removed from the China App Store rather than internationally, and nothing illegal was 'picked up' in review.
The app in question is Free Bitcoin (iTunes link).
There is no way to gather more insight, other that what Apple tell you directly. Only Apple will be able to answer your question for more information.

Ask the Apple iTunes Connect team via the Contact Us details:


  App Review
  
  Find out the status of your app and submit general questions for the App Review Team.


Ask the Apple Review team via the appeal process:


  App Review Board
  
  The App Review Board provides the opportunity to appeal the rejection of an app if you believe that the functionality or technical implementation was misunderstood. You can submit additional details to the App Review Board to help them determine if your app should be reconsidered. Sign in to submit an appeal.