This is just a personal opinion. Disabling dark theme in free version seems good on paper as it is not essential feature so shouldn’t be required to test before buying, but here’s what happens in practice. I’m using Sublime Text a lot and multiple times I thought “hey this Sublime Merge thing is probably probably pretty good because it’s made by the same dev, I should give it a shot”. Then I install it, click around a bit, but when it comes to using it in my workflow and I launch it, it looks so uncomfortable in my all-dark workspace that I always think “eh, whatever” and fall back to Source Tree.
Disabling this theme in free version still allows users to see features of Sublime Merge but prevents some users from experiencing it in practice. And I’d say experience is pretty important, that’s why I keep using Sublime Text - I’ve tried to check out Atom and VS Code (which have kinda similar feature set) but when I launch them I see how slow they are, Sublime Text just feels better so I always quickly switch back to it.