agreed. Because of the resonance in the, er, thionium(?) ion, there is significant positive charge character on the carbon atom. Perhaps it looks more like a carbocation with an empty p-orbital than a 'regular' double bond with full sp2 character? If it were a typical double bond, the orbital alignment of the nitrogen lone pair into the C(sp2) pi* orbital might not be favorable. I suspect the resonance structure giving positive charge character to the carbon atom is in play here.