The fact that the Python implementation doesn't look at write_through doesn't necessarily mean that it's not respected.  It could always write through.  Indeed, it looks like this is the case and was discussed at one point:

http://bugs.python.org/issue12591#msg141006

Also, it appears that there is a test in both cases (the test class is subclassed twice), though I could be wrong:

http://hg.python.org/cpython/file/0eeffeadaa1e/Lib/test/test_io.py#l2453

In any case, this could probably at least use a code comment.