Why dont all snakes bite when being handled, out of instinct

Why do pet snakes (and some undomesticated ones) tolerate being held, without biting? I know boa constrictors and ball pythons aren't poisonous to humans (at least I think I do . . . If this is wrong, please correct me - I'm interested in knowing), but I'm not refering to a deadly bite here, just "Hey, put me down!" I'd also be interested in knowing if venomous snakes have an "option" to inject venom or not when they bite? No real reason, just interested. Thanks.

Snakes bite when they feel threatened. (we're talking aggression bites here, not feeding bites) While a snake is only marginally smarter than an equal length of rope, they do learn which things in their environment are threatening and which aren't. Once they learn that you're not a threat to them, they don't bite. Well, most snakes, anyway. Some are just little fiends. *remembers a particular yellow rat snake* Also, certain predator-like actions, such as reaching for a snake from above, push the "threat" button and are likely to get you bitten if the snake is prone to that sort of thing. My baby Everglades rat snake will nail me if I reach into her cage, but if I let her crawl out onto my hand, she's fine. And yes, they do remember what you smell like. My snakes are much calmer with me than they are with other people. They have me filed away in their wee little brains as "warm tree that moves".

As for wild snakes . it's really a matter of acting non-threatening, and also the species of snake. I can catch a wild black rat snake and have it crawl across my arms, wrap around my neck like a pet snake, etc., but it's all in the way I move (slowly, never reaching toward the snake's head, etc.) and in the species of snake -- black rat snakes generally being very docile. I couldn't get away with the same thing with, say, a northern water snake. They're too aggressive when first caught.