I recall when I used to run half marathons they used to give you the shirt at the end when you'd finished. In some of the fun runs and rides I do now the same applies. Then you can't wear it if you haven't "earned" it.

However, I think they really like you to wear the event jerseys you pre-order, as they are usually covered with the event sponsors and the event organisers see this as a way of helping the sponsors get the due recognition. Certainly they give great encouragement for you to wear the jersey during the event.

And in recent triathlons I have been in (big ones with pretty nice singlets) they have given us the singlet at registration and strongly encouraged us to wear the singlet in the event. It's all about exposure for them on the 6 o'clock news I'm thinking.

I agree it is bad form to wear a jersey before the event happens. But other than that - wear whatever you like whenever you like!