I agree that dogs would need to eat alot of apple seeds on a regular basis to succumb to poisoning.
Apple seeds contain a cyanide compound called amygdalin (not arsenic), but keep in mind that those seeds are quite small. Canine teeth are not designed to grind food, so most apple seeds are going to leave the mouth in one piece and pass out harmlessly in the poop. The natural method of propagation for apple trees is through the stool of animals that eat apples, so the seed coat is designed to resist erosion by the digestive tract.
There is a story of a woman who loved A. seeds so much that each time she had an apple she would save the seeds in a cup,once the cup was full she ate all the seeds at once and died..