As I read discussions like http://stackoverflow.com/questions/17506515/set-active-source-to-tv-with-cec-client it appears that CEC is a work in progress so it's not a sure thing.
You could try disabling the input that it defaulted to. That idea is from a prior discussion but let me but forcefully blunt here. The product doesn't do what you want, the seller and maker have yet to answer so why keep it?
Bob
I just got the new Google Chromecast and i am able to have it turn this set on remotely via HDMI CEC. My only problem though is when it turns on the set, it doesn't switch to the correct input. Does anyone know if this set does that?
Thank you

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic