They aren't. Sure some water may be blown in by the wind but it
wouldn't be a significant amount. When the tall Sarracenia get too
much water in them they often fall over. However, water can be added
to the pitchers in amounts that will not cause the leaves to become
top heavy.
The Sarracenia have three species which seem to me to be 'the most
different'. S. minor, psittacina and purpurea, these plants have less
in common with the other Sarracenia which are basically variations of
flava and rubra themes. How different a plant is from it's next of kin
can be used to gauge the relative age of species. It seems those 3
or their ancestors split off from the evolutionary path sooner than
the other species. So it is reasonable to expect these plants to have
more differences for their horticulture.
Dave Evans