THere are many reasons why some berries are poison while others are not. Most of it has to do with reproduction. Since birds eat berries and fly away and poop out seeds, the eatible berries are cultivated by seed. Poison berries may also have very specific growing requirements and therefor drop to the ground near the parrent plant. If the plant is an annual then the seeds will grow into new plants next year.
It is also not a fast and steady rule that if an animal or bird eats a berry that it is not poison. Some animals have built up toxicity levels of the poison and are therefore immune to it. Treat berries like you would mushrooms... if you do not know for certain what it is... do not eat it.