Why not? It's not like the game/company is going away whether they ultimately win or lose in this legal battle.
This is a legal battle between the co-founders and Krafton. The co-founders were heavily involved with the first Subnautica, but for this second (third) game they were mostly just providing the overall direction for the game.
The way I see it, buying the game before September helps to take more money away from Krafton and give it to Unknown Worlds.