Why don't full array local dimming displays turn off backlight completely on dark scene?

[e22big]

Just wondering, why don't FALD just turn off their back light in dark area completely like MLED or OLED to give you true back experience? I imagine that halo effect should be worse (but then again, we already have it even fi they just dim the dimming zone instead of just outright turn it off) 

[The_russian]

This is just a guess but I think it would be easier to tell where the edges of the dimming zones are if they completely turn off, which would be very distracting. 

[dilpickle]

Yeah turning the light completely off would probably lead to even worse contrast issues.