Here is the reason
Since your application contains a web browser and allows unfiltered access to the internet, where content with mature or suggestive themes can be accessed, it should be rated 17+. Applications must be rated accordingly for the highest level of content that the user is able to access.
I'm upset and confused about this decision. First of all if I go along with it, it means that the application will become unavailable in several markets (For instance Korea and Suadi Arabia).
Secondly I don't understand why they chose a critical bug fix as the timing for a rejection of this reason. Especially for a feature that has been in the application since it's inception 2 years ago.