If there was any legal basis for this, you'd think one of the CIA, NSA, DIA, DoD, or SOC could have found a way to ban or properly confiscate the books. Or, if people at the Pentagon who were embarrassed had pooled their own money and bought the books like this, I would have been OK with that. But using public funds to ineffectively remove the books from circulation seems like an abuse of power.
The point isn't that their actions are too extreme, only that not only is the method ineffective, but they're using our money to buy and burn these books. If they HAD used a ban, it would not be a total waste of money as this is. Because they are just making people pay attention to this book. Had they not burned these, it most likely would have been passed off as yet another one of dozens of books about 9/11. Now that there has been this reaction, people know that the government is taking it seriously. Therefore, you should take it seriously too.