Iphone screen is also LED, but it's an IPS display type that's meant to look the same from any viewing angle, whereas other displays look different depending on how you view them.

Unless you're asking about turning in general, in which case the reason is light is a wavelength that moves along a plane and polarizing: mostly only allows light through along a single plane, with less light coming in as the angle from that plane increases until there is no light coming through if the wave is at a ninety degree angle to the polarization. This is because the light allowed through is equal to the sine of the angle. 1 at pi/2 and 3pi/2, 0 at pi and 2pi.

As previously stated, LCD displays polarize the liquid crystals to allow light through (changing the polarization angle by 90°, allowing the level of light through to go from 0 to 1). Because the light from that is being polarized already, hold another polarizer at a right angle to it stops light from shining through at all. A non-IPS LED screen would NOT experience it at such a level because the light is not being polarized to begin with.

TL;DR: Like I said in the beginning: while wearing polarized glasses, turning a phone 90° would not do much while looking at an AMOLED screen, but it would mean a whole lot for an LCD screen.