This show exemplifies why the Prime Directive is required because they NEVER discuss the possible implications of their actions!
What if the energy driving the volcanoes provides the heat that critical ocean plankton need to grow, so the plankton die off, the fish that feed on it die, resulting in a planet-wide famine that decimates the population?
They may well still decide to act but the fact that they do not consider the possible ramifications shows why Star Fleet must resort to a directive instead of letting people think for themselves.
Note: They broke the current lattice structures but presumably they will reform and the situation will become the same in the future, so if they really want to save these people, then need to visit every few millennia to break up the new lattices.
To have a directive that we will not help people unless they have warp drive is ridiculous even though a goal of non-interference is valid there are obviously times that interference is warranted.