They've spent a lot of hard work and money making it, they are completely within their right to charge money for it. The version that is on itch.io is an early prototype to test player reactions to the game. You did not 'pay' for it. You voluntarily donated money to the creators. £20 is not an insane amount of money. Just because they're not handing you something (that cost money for them to make) for free does not mean they are treating you 'like shit'.