Typically - when it comes to copyrighted software, any method of obtaining it for free is illegal. There might be some survey/reward sites, but you really need to be wary of them, as they will often ask for a lot of personal information, or make you download questionable programs that often turn out to be virus filled.
It is not possible to get Minecraft for free. You have to pay a certain amount of money so that you can be able to obtain it as this makes it legal. When you obtain it any other way with no payment, it makes it illegal.