Why --disable-zlib by hand?

[Rob Landley]

./configure has a --disable-zlib flag, but if I just run ./configure on a 
system that doesn't have zlib installed it notices and then dies.

Why does it do this?  Apparently it _can_ run without it.  It noticed it's not 
there.  But instead of switching off support for it, the build breaks.  
(Without even suggesting "try --disable-zlib", which seems unnecessary since 
the point of ./configure is to probe for exactly that sort of thing...?)

I'm confused.

Rob
-- 
"One of my most productive days was throwing away 1000 lines of code."
  - Ken Thompson.