Prize money based on a highscore might fall under gambling laws in some countries depending on your game (which would make it illegal in some countries). Here in the Netherlands contestants need to have a significant influence on the outcome of a contest(random generated numbers are your enemy here), otherwise it's seen as (illegal) gambling. Like others have mentioned before, you should consult a lawyer.
But hes awarding a prize, there's no input of money from the users end so its most likely not considered gambling more of a sweepstakes.Gambled usually requires money to be spent on prize awards, if entry as he states is free this wouldn't fall under this category.
But purchasing his game is not free. In that sense, what makes it different from purchasing a lottery ticket?
It's a competition, not a free giveaway. Laws get involved here.
Even Pepsi and Coca Cola prizes meet certain legal requirements like giving consumers one free entry into the competition without purchasing a Pepsi or Coke, if they mail in a letter, specifically so they can say "No Purchase Necessary" to satisfy the legal requirements.
The original posting is running a commercial business that is trying to use the possibility of money (but not the guarantee of money) to promote and ultimately drive sales of their game. Thus, it's definitely a case of "consult thou lawyer" time, because it's against consumer-protection laws in the United States to run a contest that requires payment (or purchase of a product - be it a Pepsi or an iOS app) to enter.