It is indeed a Premium-feature. Sorry, but if it’s any consolation every dollar that is paid for AxCrypt Premium goes into design and development of new features and platforms – the income goes right back into the software.
– You can explicitly sign out of the app. It remains signed in with essentially the same reasoning we have for the desktop app. See http://www.axcrypt.net/blog/leaving-computer-axcrypt/ for a longer dicussion about this. Briefly – use pin codes or Touch ID to lock your phone.
– We could do a partial finger print integration, and we may in the future, assuming that one signs in at least once in order for us to store the password in the phone but there are some fundamental differences between biometric identity credentials and the secret that is required for encryption. Also here, I’ve made a longer argument: http://www.axcrypt.net/blog/encryption-and-biometrics/ .