"Improper" Naming of Compounds?

I just got through Naming Compounds unit with my students.  We teach the students that if a compound, such as sodium phosphate (Na3PO4), starts with a metal ion then it is an ionic compound and, therefore, prefixes are not used.  These rules were developed decades ago by the International Union of Pure and Applied Chemistry (IUPAC) in order to standardize chemical names.  However, when I look at label on the back of common household items, I constantly see the names like trisodium phosphate!  Anyone know why?