My guess is that it might have a negative effect on rendering energy consumption.
In OS X you can use CoreImage to filter a view, but that has always been excluded from iOS.
The platform perception management kicks in, they can't let all the cool kids install a filter on top of all the rendering then complain that iPad and iPhone battery life sucks.
In my own Sudoku program I automatically manage the screen white point by sensing the backlight level (I'm not allowed to use the ambient light sensor, so I watch something that watches it.) but I have to do it by changing all of my colors everywhere I draw. It's not a big deal and works beautifully, until you finish a puzzle and the pure iOS alert comes up in full white and burns a hole straight through your drowsy dark adapted retinas. Then you have to go to sleep because you have a huge afterimage floating around in the center of your vision.
