Google modified the permission requirements in Android OS 6.0 (Marshmallow), requiring apps to seek location permission before connecting smart devices to Android devices. This is due to the fact that the new Bluetooth standard includes capabilities (Bluetooth beacons) that can be used to determine the phone's and user's location. Google has implemented a new criterion for location permissions to connect to low-power Bluetooth devices to alleviate worries about unauthorized devices having access to your location.
The OneKey app, on the other hand, does not use your location information and simply uses this permission to scan for bluetooth devices.
For more details on Google's permission requirements, see Google's official note:Android 6.0 Important Changes document.