Q:

Why do some LCD manufacturers use linear polarizer and some others don't?

I wanted to buy some pieces of linear polarizing films for experiments with polarized LED lights. I could not find any options in local shops nor in Europe Amazon, but some quick search told me that LCD polarizers are usually linear, vertical, horizontal, 45 or 135 degrees. I definitely knew that I don't need circular polarizers but from most materials I found online, it seems that all LCD panels should have linear polarizers because that is crucial part of LCD inner workings. So I found and ordered some Samsung Note and Galaxy LCD polarizers from AliExpress sellers with high reputation and positive reviews.
When I received the polarizers, I was surprised. They do not work as polarizers at all! They pass the light through, no matter how I cross them and even when I look through them at my LCD monitor or at a smartphone screen. When I look at daylight through one of those polarizers, they seem to be just like tinted film. But when two of them are combined or when looking through them at any LCD screen, a strong rainbowing effect appears. So, definitely they are not just a tinted film; they do interact with light in some way.
In comparison, my linearly polarized sunglasses work as expected, blocking almost all the light when I look through them at my LCD monitor (Viewsonic VP2365WB) and my Sansa Clip MP3 player and the light starts coming through if I rotate my head. But the sunglasses do not work as polarizers with my smartphone (HTC One M8S) - I can see the screen no matter how I rotate my head. Thus I deduced that my smartphone does not have linear polarizer, and also that the polarizers I bought are not linear.
So, they might be circular polarizers but I'm not sure because searching for circular polarizers on the Internet gives me results about photography circular polarizers which seem to work somewhat differently to the ones I have.
How can LCD pixels on my smartphone show up if the LC panel is not put behind a linear polarizer? How else does it work and why the polarizers are needed at all?

A:

It seems to me that you have been shipped tinted film.
To test for linear polarized film rotate one piece in front of the other. When lined up as laid out in the original sheet they should pass light. When rotated at 90° they should block all light and appear black. (You obviously know this.)
To test for circular polarization:

The simplest test for whether a polarizing filter is really and truly a circular polarizer is very easy. Look through it into a mirror. [In one orientation] light is reflected off the mirror and passes back through. There is some darkening (about 1-2 [camera f-]stops with each pass through the filter). There is very little color cast visible.
Now reverse the filter. Now the mirror image of the filter is very dark.
If you try the same test with a good linear polarizer, there is absolutely no difference on reversing it - the image looks identical, and with minimal color cast.

Source: Beware cheap "circular" polarizers. Text in [ ] are my insertions for clarity as the article is about camera filters.

