I think its fair simply because they've PAIDED you to learn it and produce a product. While you were never sent on any formal training and you had to learn it yourself without any help... that was training for two reasons 1) you were allowed to learn persumable at your own pace and probably to make mistakes along the way and 2) they paid you during that period of time. If you'd prefer not to share that knowledge then I think its only fair that your employeer ask for their money back during the period of time that you were training yourself.