I think that the confusion comes from the fact that when children are being potty trained they wet themselves much more often than they soil themselves. The reason for that, of course, is that most people urinate considerably more often than they defecate. It is true that the goal of toilet training is for children to stay clean and dry, but that shouldn't be where the initial attention is focused. Before children can stay clean and dry they have to learn to do their business into a toilet or potty. And since children urinate much more easily and much more often than they have bowel movements, it is easier to train them to urinate into a potty.
Children learn from their experiences.
We can help them significantly by pointing them in the right direction, showing them what we want, and providing them with the opportunity to do what we ask and figure out what we're talking about, but the no matter how many times you show a child someone else using a toilet, he won't fully understand what it's about until he experiences it himself. Essentially I'm saying that for a child to learn to use a toilet he has to experience using a toilet. It's really not. Since toddlers don't really care where they go, just like an untrained child can have an accident in his pants, so can he have an accident into a potty. All he needs is the opportunity.