You have raised two not completely uninteresting questions:
>Evolutionarily, there would be nothing stopping a plant from trapping for
>consumption any and all non-pollenating insects as long as that did not
>affect their target pollenator species.
In fact, there are many examples of flowers which protect themselves from
crawling beasts (which would cause self-pollination more frequently than
flying guests which change their "host" individuals more frequently) by
means of adhesive secretions on stems, inflorescence axes, bracts,
pedicels, calyces, etc. In some cases, these plants are dangerously close
to carnivory (at least the wingless taxa thus excluded do frequently not
survive their trip to these inflorescences). It may be noteworthy that in
the case of glandular bracts or calyx lobes it is mostly the lower (outer,
abaxial) surface which bears the glands, and the supposedly more
"primitive" representatives of the clade including so well-known carnivores
as Nepenthaceae and Droseraceae, viz. _Drosophyllum_, and to some degree
_Triphyophyllum_ carry their stalked glands also on the *abaxial* leaf
surface. We cannot (yet?) reconstruct the course of evolution with any
certainty in these cases but I could imagine these trapping leaves to have
been transformed from former adhesive inflorescence structures (NB:
_Plumbago_, a suggested non-carnivorous sister taxon of Nepenthaceae, does
have glandular inflorescence parts, and of course it has plumbagin,
and...?).
>why haven't carnivores evolved to more directly
>exploit the pollenating habits of insects? Or do the pitcher plants do this?
Most cps have traps which are resembling flowers in many respects. In
_Cephalotus_ and _Nepenthes_, the traps are much more showy than the
flowers (at least to my eyes).
Kind regards
Jan