I agree that reading from a file open for write should be forbidden, no matter whether ZipFile was called with fp or a name.

Actually, it is not yet forbidden, and two of the tests in the zipfile.py test suite do actually rely on this misfeature.
The first chunk in the patch http://bugs.python.org/file24624/Proposed-fix-of-issue14099-second.patch contains a fix for this bug in test suite.

OTOH, decompressing several files for a given zip file simultaneously does not sound that bad.  You know, with all the current file managers, people look at a zip as if it were kind of a directory.