Simply because that’s where 90%+ of the fighting took place.
Therefore I think it’s a little disingenuous to claim that Unionists when they behaved the same or worse were simply imitating the CSA."
No, it's clearly a matter of perspective.
First, for example, you say 90% of the fighting took place in the South, but I would dispute 90% based on the fact that Maryland, Kentucky, Missouri, Kansas, Oklahoma, New Mexico and West Virginia were all Union states, and all saw significant fighting, especially in the war's early years.
Indeed, Lincoln remarked that had he lost Kentucky, the war would be lost, so it's hard to overstate the importance of those battles.
Second, I would say that 90% of Union troops, even in Confederate states, lived off supplies from their own railheads, not "living off the land" from confiscated Confederate supplies.
Third, to the degree that either army "paid for" their requisitions, the Union did so with real money, Confederates with near-worthless Monopoly-money.
Fourth, the Confederate Army was almost never entirely self-sufficient -- even within the Confederacy they had to beg-borrow-or-steal food & other necessities from local populations.
Once outside the Confederacy proper, they were at least 90% dependent on "living off the land" of Union civilians.
Finally, a major if not only purpose of many Confederate raids North (Morgan 1863) was to secure supplies and destroy anything of military value -- i.e., railroads & bridges.
Therefore it is totally fair and accurate to conclude that Sherman's actions in Georgia were simply bringing home to the Deep South things their own forces had been doing "up north" for years.
Morgan's 1863 raid:
