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It's a curious topic, as I am working on precisely this functionality for PolarCOM right now.
According to NMEA "true" wind is defined as a wind that would be felt by the same vessel if it was in the same place in the water, not moving relative to the water.
I.e. "true" wind by that definition is "true" in relation to water but NOT in relation to the ground. This does make sense, since effect of wind on any vessel is only important relative to the vessel movement in the water. Unless it is stuck on the rocks or something.
In any case, this is what the standard says and so any compliant device has to use only heading and speed through water. Although SOG may be a logical choice (and a preferable for some), compliant devices won't do that.
Incidentally, PolarCOM will give an option to use SOG/COG for "true wind on land" kinda thing precisely for the reasons you describe.
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