This is a great question. I would say that a preschool should introduce some sort of learning to increase brain capacity and facilitate learning. Children even at the age of two and three are capable of learning greatly. For example, three year old children are able to recognize shapes, classify objects, understand cause and effect, and many other things.
In light of these abilities, why should we not stimulate learning and growth in other intellectual pursuits? It would be irresponsible for educators and parents not to help children grow in these areas.
Moreover, all of these educational exercises can be extremely fun. In other words, fun and learning can be one and the same. In light of all these points, all adults should know that children are able to learn greatly and can take leaps and bounds in learning.
I feel at any age you can make education fun and fun educational. Does that mean you need to sit your two or three year old down and start prepping him/her for their SAT's, no, but why not make every experience in your childs life an educational one?
Children learn through hands on, inquiry based instruction. You can say "No, hot," all day long, but until they touch and can put the word with feeling, the activitiy, the ownership of their learning, then they are mimicking you, not learning for them.