I have every sympathy. Being accused by an almost-faceless corporation of what amounts to willful misuse or crass stupidity with a piece of equipment which to them is virtually worthless and to you is a significant investment of your own money is - at best - insulting. While it's inevitably true that some users do cause screen faults because they simply are not careful with their equipment (or other peoples') and Apple are right to be wary of warranty claims under these circumstances, to be tarred by the same brush as part of a broad policy is fundamentally wrong.
However, while not excusing Apple in any way, there are two pieces of information pertinent to this issue which you do not have and which they do: the torsional rigidity of the display panel (ie, how easy or otherwise it is to break) and the proportion of MacBook Air models sold which have been reported to suffer one or more screen fractures. These two pieces of information tell Apple what the balance of probabilities is that a screen crack may be caused by component fault or mishandling, and likely is influenced by where the crack has appeared, and also whether or not there is a design weakness with the product.
In the latter instance it seems reasonable to believe that some structural rigidity was compromised for weight and thinness, but with a product that has been on the market for almost 4 years now, it is highly unlikely - most improbable in fact - that the level of faults reported would show a failure of design or a proclivity to structural weakness. There has been a lot of time to correct an issue such as that with successive models, and there has been no correction.
In terms of the screen itself, if the crack in the screen fits the profile of a mishandled system, they will deny warranty cover in the interest of both themselves and other customers.
Put in that position myself, I would be as upset and angry as anyone else, but in the end neither that, nor the 'I-will-never-buy-another-Apple-product-EVER' attitude will help get the problem sorted. I would call Apple Customer Service and explain what has happened and why you are upset and feel that you are being unfairly accused of damaging your own property, and have basically been called a liar by staff in the store whom you had approached to help you resolve a problem that was not of your own making. Further, that while you know it won't impact on Apple's bottom line, this incident and the failure of the product makes it very difficult for you to justify any further purchase of Apple equipment - which is a shame because you've been a committed user for years because the products have always been better suited to your needs than those of the competition. Ask them if they can help you resolve this problem, and if necessary ask for it to be escalated if the first respondent cannot.
You may end up with the same answer, but you may not. Apple does have legendary customer service as regular surveys have shown, but it isn't flawless.
Again, if it were me (and who knows, one day it might be because I'm typing this on a MacBook Air) if that didn't get me a satisfactory outcome, I'd buy a new display panel off the internet and repair it myself - closer to $200 than $800. And I might just consider asking a lawyer to mutter the word 'defamation' into Apple's corporate ear into the bargain too.
