I am closing this as some combination of wrong, inapplicable, out-of-date, and postponed.

1. In 3.1, ssl *is* documented as optional in the sense of dependent on an external library. "This module uses the OpenSSL library. It is available on all modern ... platforms, as long as OpenSSL is installed on that platform." For bz2, there is "This module provides a comprehensive interface for the bz2 compression library." If the library is not there, then the module obviously cannot function. Tkinter depends on tcl/tT being installed. I believe some crypto modules also require possibly absent libraries. (The PSF/Löwis Windows installer nicely provides all.) 

If someone wants to review the docs for all such dependencies (and perhaps whatever build docs or help strings or comments there are) and propose doc revision, that could be a separate issue.

2. I think the current default build process is right for most users.

3. Except for the PSF provided binaries, building is ultimately out of our hands. Distributions do what they do. I presume individual persons and organizations can patch the default build files to be stricter if they wish. If they cannot, neither can we ;-).

4. This seems to have become pretty much a non-issue. The OP says he has no further concrete interest because "It's been a while since I had a computer without these libraries...". I am suspecting this is pretty much true for everyone who might otherwise care enough to provide a patch.