The general principle here is, as Mervin stated, feature discovery (or accidental discovery).
In this specific case, it's an edge case that we really didn't design for explicitly. The more general case is for people that don't have enough rep to vote and allowing them to play around with the interface and discover the capabilities of the site and the features they have to look forward to.
We have a long way to go as far as consistency on this stuff, but as a general approach, we prefer to expose common functionality even if you can't use it. In some places, we will let you know why, in others, it's a way to gather statistics on how much people would use a feature before they can actually use it. And, in other cases we will try to convert an attempted up vote to sign up or some other engagement if you are an anonymous user.
We prefer to hide uncommon functionality that can't be used to keep the interface on the simple side.