You make a valid point. Patreon contributions are entirely voluntary, and fans choosing to support the developer are doing so knowing full well that the game is offered for free. It's all about personal choice—people are free to support or withdraw support if they feel the development process doesn't meet their expectations.
It's also important to remember that crowdfunding, especially in creative projects like game development, is often unpredictable. Just like tipping a busker, as you said, you're not paying for a guaranteed final product on a set timeline—you're supporting the creator and their process. And in the case of Summertime Saga, it's clear that the developer hasn't promised anything beyond what they lay out on their Patreon.
In any case, if people aren't satisfied, they have the option to stop supporting. The real key is transparency, and from what I've seen, the dev has been clear about the nature of the project.