I do believe snakes can become tame to a certain degree, because when looking up tame snakes do fit into some of the listed definitions and adjectives listed.

As Vypyrz said most likely they register things as a threat or no threat. Food or no food. Their minds are simple and their learned behaviors are minute if any at all.
Most all is instinct. And they have triggers as far as when to go off that instinct,

Just like a pitbull, snakes can be tamed to a certain level but in certain situations they can snap just like that. Because beyond our efforts to "domesticate" and rid them of their instincts...the instincts will always be there harboring and chances are at some point something will trigger them.