This is working at intended. tox is about reproducible build, hard coding to full path seems from this point wrong as it's almost bound to fail on other peoples system. Discovery on Linux systems should be via having the pythonx.y format as specifiad in
http://www.python.org/dev/peps/pep-0394/#application-to-the-cpython-reference-interpreter. tox follows this logic for interpreter discovery, especially for pre-defined build environments.
That being said I know this worked until now. So in that sense it's a regression. I would say though that your use case is the edge case, if you want to use it like that define custom tox environments, e.g. py27k instead of py27. In general, for people who follow the python discovery system enlisted in that pep, this change should help them avoid invalid configuration and bad suprises.