Most hits when Googling make no real attempt to explain why writing in a mirror doesn’t appear upside-down. This article is interesting, and comes close. I'm still not sure, though.

http://www.xs4all.nl/~johanw/PhysFAQ/General/mirrors.html

Interestingly, if you consider up, down, north, east, west and south, and point in each of those directions and observe where your virtual you (in the mirror) is pointing, he points the same way as you, but reflected in the direction perpendicular to the plane of the mirror.

But I still don't get why writing is reversed, but not upside-down 🙁