Why on Android the app costs?
Oh, if you want, I can tell you. Initially, android is harder to make. Secondly, the money is used to prevent the minors from downloading the game. You see, android is very common among the minors, so if there were no payment, they would be able to download the game freely, which goes against of the age restriction.
To add context to Intenselog's comment. Websites featuring adult content are required to ask if you're 18 or older. Itchio however does not. Requiring the app be purchased acts as a pseudo-age verification. In the event they get sued they only need to bring up the receipt to show they did their due diligence. The reason it is 5 USD is due to free trial exploits. Most banks and whatnot use a hold charge method that typically ranges between .99 USD and similar. By making it 5 USD they can bypass anyone using these exploits. They were fairly common in the early 2000s to access trial based membership adult websites. They have yet to be patched.