Unknown sources box is Grey can't check. topic

So I just switched to t mobile and got a new note 3, coming from a Verizon note 2. I went to check unknown sources box and it's Grey and can't be checked. Does anyone know why? I even looked on my wife's new gs4 and unknown sources can be checked. I'm planning on rooting, I've just been doing my research on the whole knox bs. I've had a lot of android devices starting with the original DROID and they've all been rooted custom roms and tweaks. I'm no expert, but I know my way around android devices. Thanks for the help.

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