Slitherine

Why do units that must be adjacent to enemy units in order to attack (e.g. Infantry, Tanks, etc.) have a range value of 0 in their units stats instead of 1. Artillery units count adjacent hexes as "1" when calculating their range. It would be more consistent to count the first / adjacent hex as "1" for all units in the unit stats.

I think it has to do with return fire. A unit attacking with a range of 1 is not subjected to return fire from the defending unit while a unit with a 0 range has to take return fire. That's my guess at least.

Makes sense to me. I'll try to attack with a "range one" unit and see what happens.
BTW, do you know what unit(s) have a range of 1?

Off hand, the only one I can remember is the StuG, a motorized artillery unit available in the early year scenarios. I believe all of the other artillery and AA have ranges of 2 or 3.

Yes - artillery units have range 1+ and cannot be retaliated against though may need to be adjacent if they have 1 range.