The EXA100 will be able to turn on/off in sync with your TV when connected via the eARC input. If your EXA isn't doing this, we would suggest trying the troubleshooting steps below:
1. Ensure that all relevant CEC settings are enabled on your TV
Most TV will have CEC settings available, which will allow you to enable or disable power control between the TV and an external speaker.
2. Ensure that TV Power Control mode has been enabled on the EXA100.
You can do so by following the steps in this FAQ.
3. Ensure that the EXA100 is connected to your TV via the EXA's eARC input