setuptools: data files included with `bdist` but not with `sdist`

I've got a setup.py file which looks like this:

#!/usr/bin/env python
from setuptools import setup, find_packages

setup(
    name="foo",
    version="1.0",
    packages=find_packages(),
    include_package_data=True,
    package_data={
        "": ["*"],
    },
)

And a package foo which looks like this:

foo/__init__.py
foo/bar.txt

When I run setup.py bdist, the bar.txt file is (correctly) included in the distribution… But when I use setup.py sdist it isn't.

What's up with that? Am I misunderstanding the meaning of package_data? Or is this a quirk of setuptools?