2 main things really
1 - by 'the market' - the news usually talks about the DOW. You can buy DOW futures, the first point is that no amount of buying DOW futures would move the DOW itself. Rather the DOW leads the futures & not vice-versa. Therefore to manipulate the market, you would have to buy the underlying stocks.
2 - that the government would get it's ass handed to it on a plate if it tried to manipulate the market by stepping in and buying up stocks at one point in the day and selling out later in the day. Professional traders and Program Traders would ream them.
They'd be looking at billions in potential losses every time they tried to manipulate the market. The sums required are vast and the more the market needs to be manipulated, the more you'd need to spend to move it in the opposite direction.
