I believe it's a safety precaution. It doesn't happen often but sometimes you can get calls while you still have the phone at the ear (it happened to me only twice during the years).
If the phone rings at that moment it could be really dangerous f...
For those coming over from samsung devices and are using Chaton (like me), Play store incorrectly says the app is incompatible with Xperia Z.
I've found and tested the latest Chaton apk and everything is working ok.
So for everyone missing the ...