I guess it is just that these GUI toolkit people usually are trained to
deliver good looking and pixel-precise artwork and in consequence might have
some difficulties to imagine how it can be "pretty much impossibe" for some
editor to know how many lines it is actually displaying (or anyway that such
editor would still want to use their scrollbars).
No, sadly, it is really due to dogma: your above explanation would make
perfect sense, but in practice, even after showing them our problem,
they still insist that it would be wrong for them to remove the chunks
of code that enforce the "scrollbar metaphor" by preventing the client
from getting the events that indicate that the user wants to move the
thumb's bottom further down than the scrollbar's own bottom.