If you look closley, if you wanna download, lets say the dmg file, it says "if you pay 1$ USD or more." The reason why it appeard as an optional donation is probably to make it so you can pay extra if you want to donate.
Just figured out that when you put a price tag for your game it says to pay "$__ or MORE." I think the creator thought they would also make the web version paid? I don't really know.
