jdybnis: I think the cheeky answer is to be found in r5rs wrt. the disjointness of types. however, evaluation as boolean is polymorphic, such that anything which is not #f is coerced to #t. so your code can be written as:

(define (breadth . trees)
 (or trees
  (cons (map car trees) 
   (apply breadth 
    (apply append (map cdr trees)))) 
  '()))

but the perhaps-valid question you could then ask is: why isn't '() coerced to #f, and conversely why there isn't a coerce-to-list form for booleans. dunno. MIT scheme treats them the same, so perhaps they didn't see the logic in it either.