This is the core devouring ability of the Vasari exiles, which can destroy the entire planet and gain access to the planet's resources and strategic resources, you can hover your mouse over the planet, and the information card says "The planet has been permanently destroyed".

naw, its what they want to do. Its what that specific faction does. They take what they can get and they move on. Its not "unreasonable", its literally a core part of their behavior. Is it smart of them? Maybe or maybe not, but it is what it is and they did get a lot of resources for it.