Is this really appropriate? I know Octave is supposed to be an open-source MATLAB replacement, but if the user doesn't explicitly ask for Octave I don't think it should be re-tagged.
MATLAB and Octave are not always identical, although they often strive to be. An answer for one does not necessarily work in the other. Adding an Octave tag often adds it to the post's browser title, and makes it seem like an Octave solution is acceptable.
This seems wrong to me. Should I go back and remove the tag on all these posts? There are lots.
If this practice is acceptable, then the tags should simply become synonyms of each other.
Update: The tag was not being applied to all new matlab posts, as originally described, but many of them which were thought to be cross-compatible with Octave. See discussion in comments below.