Seems like a completely arbitrary limitation.
Quite a few people probably use liters of oil,
or in my case: liters of LPG.
Of course, one can just convert to m³ of LPG, but it is a lot less intuitive, since you usually pay by liter.
No idea what units other heat sources use (district heating, pellet, …)
EDIT: Just noticed that ft³ is now allowed, too. But not liters for LPG. Could that be added then?