See PEP 486 (https://www.python.org/dev/peps/pep-0486/). This is intended behaviour, as it is assumed that explicitly specifying a Python version means that the user had a specific Python interpreter in mind.

There are also technical issues - if the shebang says #!/usr/bin/python2, and a virtual environment is active, how do we know if it's a Python 2 or 3 environment (without launching the interpreter, which is a significant extra cost for the launcher)? It would definitely be wrong to launch a Python 3 interpreter in that case.