It's normal for an LCD panel to be fragile. I'm not sure why the screen would crack, but if it's being pushed past the swivel's limitations, then instead of turning the swivel, you're essentially twisting the television.
If it's as you describe, it would likely not be covered under warranty, but the service department would be the one to give a definitive answer. I'm not an authority on the service matters.
I'm sorry to hear that this happened.
We bought this tv for my dad t'giving of 2009. It has a swivel base and he turned it a little past where the swivel stops. The base of the tv moved a little and CRACK! The screen is now shattered. He had a tech come out and look at it, but they have refused any warranty liability.
Is this normal for an LCD? I have a much cheaper model in my Magnavox, but that dang thing has been moved back and forth several times as I have changed various inputs over time. I've moved it a lot more than what he did and (again) it was a MUCH cheaper tv.
We bought a Samsung because we thought it was the best, but maybe there's more to being the best than having the best picture. Part of it would be standing behind your product.