Subtractive manufacturing has been the way of the world for a while, but additive manufacturing, which is synonymous with 3D printing, has disrupted all of that.
If extrusion-based printing is considered additive, then can we call resin printing additive?
Resin printers start with a resin bath and then essentially "subtract" the material it needs (via UV light). This is just a matter of terminology, no big deal.
Resin printing (aka stereo lithography) was actually invented before (FFF/FDM) filament extrusion printing. The term 3D printing was more or less created as a generic way to name and describe both along with a handful of other methods.
It is additive because you add layers to other layers to build the part rather than carving up an original solid block.
Resin printing is no more subtractive than removing the tip of the filament in FFF is subtractive.
It really took me a little while to understand why you thought that it was subtractive, but I can now see your point. Which is, that seeing as the resin model is created within the tank/bath, you are saying that the bath is analogous to a block of aluminium which is whittled away, using CNC, to make a 3D model. The resulting aluminium model was once within the block of aluminium, as was the resin 3D model - therefore, you posit, resin 3D printing is subtractive.
The more I thought about it, the more it seemed as if you were, actually, correct.
However, then I realised that we were neglecting the hardening process. It is the hardening of the resin that is done in an additive manner, in order to slowly build up the model, therefore the process is additive.
You aren't removing/subtracting non-hardened resin from the model (which would result in a non-hardened wobbly model), but instead, as user10489's answer states, you are adding hardened resin layers.
