Yes, freakie and lynn have it right. Snakes aren't like dogs, and dont know that they should not bite the hand that feeds them. They can't be trained to not bite in the same sense as a dog, but they can learn to not see you as a threat through interaction.
Expecting any snake to not bite you when your messing with it is pretty rediciulous. If it sees you as a threat, it will either fight, or flee. This does not mean that one "test" of yours, with the snake deciding to flee, does not mean that the snake will always choose to flee rather than fight.
I think health and background are far more important than temperament when it comes to snakes.
I suggest you throw the whole personality-specific side away. Understand that they are working on some very simple instincts, and they always have the possibility to bite you, but also always have the possibility to be a calm and "sweet puppy pumpkinnoodleface".
You just wont really know their temperament for sure until you have that animal home with you, settled and eating.
[edit]
I just saw the kicking dog comment, I meant no comparison with my post.