
You're assuming that their normal rotation carries all areas of the surface
into sunlight.  Not so.  Even on Earth, each pole gets several weeks without
sunlight in mid-winter.  Pluto and Charon have much more extreme axial
tilt and a much longer orbit.  Pluto's north pole, for example, gets over
a century of darkness followed by over a century of perpetual light.

At the moment, we're in luck -- Pluto and Charon are just past their
equinox, when the Sun was just on the horizon at both poles (of each).
If we get probes there soon, only the immediate vicinity of one pole
(on each) will be in long-term shadow.  This will get steadily worse the
longer we wait.