Actually it seems that the Tesla superchargers had everying implemented to charge any vehicle, but lacking a payment process would refuse to do so. This became apparent when about a year ago there was some issue with the authentication module at some supercharger in Germany. For a day or so it would happily charge any brands car for free. This quickly spread via the internet and soon there was a wide variety of non-Tesla cars charging there.

One still wonders whether this just was a strange bug or some undercover test arranged by Tesla :)