The only logic I can come up with for having the TAF in degrees true is so that pilots don't have to remember which forecasts indicate true, and which ones indicate magnetic. Clearly, an ARFOR or GPF couldn't indicate magnetic due to the large forecast area often covering multiple isogonals of variation. So rather than having one rule for ARFORs etc and another for TAFs, people can just remember the distinction turban mentioned - if it's on paper it's true if it's spoken (i.e. ATIS or by ATS) it's magnetic.
I do tend to think the METAR would make more sense in magnetic though! Maybe the line gets too blurred with TTFs....Who knows. If it ain't broke....She'll be right mate
