The problem is that people who run into venv will think that it is
virtualenv made into part of the standard library.  Thus they
will have a great expectation that there will be a -p option to
specify the python version that they want and that they can get 2.7.

As Josh Billings (American humourist) wrote:
"It ain't ignorance causes so much trouble; it's folks knowing so much that ain't so."

A 'this is not virtualenv' message would be useful.