If you don't have to pay to get in then it's not a lottery. I suppose you could call it a contest, but there is nothing wagered like a lottery. I doubt any court would find otherwise.
I *really* dont want to side with OP, but is some terms I think he is right. Bitcoin mining does seem more like a lottery or contest in comparison with actual gold mining.
And you DO have yo pay to play. its called electricity. About a month and a half ago I turned off all ten of my 1TH/s miners. It made no sense to mine using electricity when the value of bitcoin was/is dropping. I was "paying to play" and i could not afford it any longer. In some respects its not much different than if you go to a casino to play blackjack and you do well and profit, take a break, come back after lunch to play and you start losing. Are you still gonna play??? Are you still gonna mine?