> Surely the build system should know how to correctly name the wheel it builds.

It's probably worth mentioning the specific problem that motivated pip
to start doing this.

It used to be standard, and is still quite common, for setup.py
scripts to contain stuff like:

    install_requires = [...]
    if sys.version_info < (3, 4):
        install_requires += [...]
    if platform.python_implementation() == "PyPy":
        install_requires += [...]

    setup(..., install_requires=install_requires)

This kind of logic in setup.py worked fine in the old days when all
you did was 'setup.py install', but then wheels came along

and
retroactively turned lots of setup.py scripts from working into
broken. The problem is that with this kind of setup.py, 

But it will take a while for existing setup.py files transition to
using those, and in the mean time pip can't assume that a random wheel
generated by 'setup.py bdist_wheel' has accurate Python tags.

Hopefully new legacy-free backends will get this right from the start.