The principle is fairly basic. The function of many of the arguments on this site is to prove that the Earth is flat. If it can be proved that the Earth is flat, then the government would surely know that it is. Ergo, they would be lying.
In normal circumstances, the presence of a motive for the conspiracy would be essential. However, it can be bypassed simply by proving a necessity: if the Earth is flat, there is a conspiracy. Simple logic. It can be fun to speculate on the potential motives, even if you're just messing around, but ultimately it brings nothing to the debate.
I recognise that saying "the reason is unknown" is not the most satisfying conclusion. However, whereas in most cases a motive would be a necessity, in this case it is not, as proof of a flat Earth would also prove the existence of a conspiracy. Therefore, discovering the motive could safely wait until proof of flat Earth is found.
Indeed bugman. The problem is, you're going to be ignored just like me, Cinlef, Erasmus, Unimportant, EnragedPenguin, 6strings, and many other people who have been stressing this.
It's understandable that people would demand a motive at first. Modern society demands explanations for all actions. At first, I was annoyed at the dismissal of the need for a motive, because it felt like a cop-out. However, I thought about it, and realised that it is not an imminent necessity. I would presume that most casual visitors are just unwilling to apply to much thought to the matter.
To be honest, when I first came here a couple of months ago, it was for the sole purpose of trolling and winding people up. Once I understood the nature of the forums better, I was kind of won over, and that's when I started applying actual thought to the matter. I think anyone who sticks around for more than a fleeting visit should ultimately be able to get hold of the principles.