See page 166 of Fox's _Biology and Diseases of the Ferret, second edition_.
Plant proteins in amounts too large raise urinary pH, which should be about
6 in a ferret due to them being obligate carnivores.  (I can just see
people running for pH strips now.) Corn is ESPECIALLY bad for raising the
pH so the original poster was not just making this up; the person probably
has a copy of Fox's book and as a result knows that corn is a no-no for
ferrets in amounts smaller than exist for some other sources of plant
protein.  There's more than simply the nutrition accessibility aspect.
 
Beans: I'd guess it would depend on the size of the beans used and on them
being changed on a regular basis in case anything nasty may be growing in
them.  (Molds are very dangerous sometimes.) Some beans I could imagine
causing strangulation if ingested, and if they did make it to the stomach
they might take a very long while to digest if they digest at all so might
cause a blockage.  This is guesswork only.
[Posted in FML issue 2863]