Preemptive incompatible with POST ... why?

Folks:

I'm developing a utility function for clients that POSTs
a request to an HTTP server and converts the response
into an appropriate data structure.

I don't have to interact with external event dispatchers
and I don't think I should have to use libwww's event
dispatcher.

Except that it doesn't work unless I do.  That is, if my
set up includes HTProtocolPreemptiveInit(), when I do
HTPostFormAnchor(), my posted data is never posted, and
the web server times out, treating the content length as
zero.

So, my question is, why?  Is there a synchronous way to
POST form data with libwww and block until the response
comes in?

Thanks.
--Sean