There are (several) guidelines that abolish certain abbreviations because misinterpretation can kill, for example http://www.ismp.org/Tools/errorproneabbreviations.pdf

I cannot find a guideline w.r.t. Mg, but it can easily be confused with milligram and microgram. That may be the reason to use this weird construct ('ton' seems a better term, but it is ambiguous, too)