Looking at the U.S. systems ATIS is magnetic north referenced (Airman's Information Manual para 4-1-13) The automated surface observing system (ASOS) and other automated systems produc ea true north referenced output for METAR and SPECI reports, 5 minute observations and the daily wx summary. When the asos is fitted with the ground to air radio feature it converts to magnetic before broadcast. Not sure what the digital ATIS in oepration at some locations over ACARS provides.
Seems the problem might be when whoever created METARs and internationally standardized them maybe they forgot who used them. Then the automation and automated wx observing engineers just provided what was demanded at the output.
references: www. faa.gov/ATPubs/AIM
www.nws.noaa.gov/asos/magwind.htm