Of course, of course. It seems so obvious when you point it out
Having other chargers (even if you had proper access to databases) would make a mess of it for the majority of trip calculations.
So is there a compromise? Perhaps, although I have no help in terms of implementation, you could include destination chargers at/near the destination and/or only when a supercharger isn't in range and the computation would otherwise fail?
Where it failed to me was when I attempted to plan a round-trip, and perhaps that's just outside the scope of the service. And that's fine. In reality, I can be left to my own devices (or plugshare) for finding a slow charger at the destination.
Overall, great job. It isn't (yet) perfect, but from my perspective it is the best route planner available currently.