Research tells us that children construct their learning and that language is no different. Children are very capable of using the world and people around them as tools for building their understanding. So for a child who passionately reads picture books and writes on whatever they can get a hold off, going to school and being told that now they are going to learn to read and write is rather ludicrous. They are already reading and writing! School is in place to help children continue to construct their understanding, not to start from scratch.
The three year old often watches me while I do work for grad school. He constantly sees me reading and writing notes. Yesterday he worked beside me as I read and this is what he wrote. He wrote from left to right, highlighted the important part of his writing, and even drew an arrow to show him a very important piece. This is writing. And excellent writing at that.
This, on the other hand, is not writing. This is a rote practice activity in which his preschool teachers want him to correctly form the letter symbol. This has no meaning to him. When I asked him what he wrote he said, "nothing, it's just some letters". He's right of course, the note that he wrote was full of meaning and written for a purpose, this worksheet was not.
In our haste to conform to standards we often forget the child. We forget that they construct their knowledge based on the authentic "whole". It's important to know letters, but it's more important to know them in a real and meaningful way.