Obviously if the snake refused worms and fish and took live mice that is something that it eats in the wild, snakes are instinctual and they eat what their brain tells them to, there's no decision making process. So clearly this is part of it's natural diet.

I think the role rodents play in a garters natural diet is highly underrepresented, every single wild garter I have ever caught has taken unscented pinkies usually on the first try. If they didn't eat them in the wild they wouldn't do that.