First, this was a hack for myself. Second removing the font-families is intended to be used together with remove fonts. This was because having font-families prevents kobos reader to change fonts. And even if you don't remove fonts in addition,, that is not a bug,, as the code still does what is said. You only have a slightly larger epub with fonts in it which aren't used. Maybe someone even wants this for reasons I can't imagine
I will look at it today or tomorrow, since I think I have to modify the regex slightly to catch all cases.