The difference is that what you're referring to is millions of people spending a couple of dollars on something willingly. If the government was covering up for a flat earth there would have to be billions of dollars put into altering photographs, changing the sky so everything looks round, subliminal messaging, etc. Where does all this money come from and what benefit does the government get from it?
Furthermore, if there was such a massive organization devoted to this deception, how come no one has let the word slip? It seems like with thousands of employees someone would find it kind of' interesting that the Earth is flat.