Default MHL questions

1. It seems like tablet screen always has to be on. Why? If I want to watch tablet content on TV, why should tablet screen be on?
2. I have mhl cable which I bought a year ago for my xperia z phone. Mhl 3.0 supports hdmi-cec but I cant use it on my Samsung TV. It doesnt recognize z2 as an hdmi-cec device. Do I need a new cable to support new mhl 3.0 features?
3. Black bars on TV left and right side. It would be nice if it is smart enough to show movies in full screen. But seems it is not.

Sent from my SGP511 using Tapatalk