I think you have to consider that this may well indeed come down to price/quality.
I work in manufacturing and quality and can understand the basics of things like this.
If you go to someone, sit round tables, talk about products, prices, quantities, tolerances etc etc, you come up with a balance.
If we make up a scenario and numbers, we could say a screen costs $40, but we need to get the price down to $20 We can do that in a variety of ways, what it's made from, number of parts, the spec of those parts, BIG orders obviously mean money savings in bulk quantities all the way down the line.
However, there will come a point where price affects the quality control, or rather what you are willing to accept. What gets through the net, so to speak.
I'm sure we've been to high end food shops which have high prices and the food looks great, Apples are all big and shiny, Banana's and long and have no black specs, Oranges are sweet not sour.
This is because they pick and grade the items more and select the best, rejecting product that does not meet their high standards, but the effect is that the price is higher to guarantee this quality.
You may go to another lower price store, and yes, you may find some equally nice Apple's Banana's and Oranges, but you will also find a lot poorer quality ones as they are not so picky and don't reject the slightly poorer ones.
There is no difference to this concept as it is with things like LCD panels for iPad's You set a price which reflects the construction of the item and also what you let through the net as acceptable.
Apple and the screen makers obviously agreed this balance. Yes, they could reject iPad's, or the screens, but then the prices of the good iPad's would have to go up to offset the lost profits from the rejected ones.
They can just decide "This is good enough for most people" and then just replace them if a few % of people are unhappy.
This is probably far cheaper for Apple, Just replacing them for customers who complain, than it is, by rejecting them at the factory, as hey, some customer may have accepted it anyway, or increasing the overall quality control which again would mean higher costs, and stopping a % of iPads reaching customers, who, you never know, may have accepted them.
Making products of this quality, and using the customer as a kind of quality control may well work out much cheaper and more effective at shifting large quantities than creating an artificial accepted quality standard at Apple and rejecting items before customers even look at them.
Not the route I personally would take, but I know we as a company have actually done this ourselves. We make something and we know it's out of tolerance but it would cost a lot to remake so we ship it anyway, and we may find 9 times out of 10 the customer accepts it anyway and we get our money.