This makes logical sense, but I think there is a sunk-cost psychological aspect that people tend to handwave away. For example, if you start supporting a game for say.... $10 / month, and you support the dev for a whole year... $120 has been given. If at the end of the year, if you are not happy with the progress over the year, then you can of course stop, but you still "lose" that $120, especially if the dev ever cancels the project or takes a long hiatus, and its not like you can get a refund really.
Now I know the whole idea of Patreon is to that you are not "buying" anything, and so you should have no expectation of getting anything... you are just supporting someone in the hopes that they'll continue to do something that results in something you want. But you still spent money, and if at your "breaking point" you feel you didn't really get anything good out of your investment, then there will be anger, disappointment, etc. And some people will keep paying because they already paid so much money already...surely this will produce results eventually, right?! (*cough* Star Citizen. *cough*)
So yah, on paper, you can just shrug and cancel whenever, and you shouldn't have expectations, because that's the logical thing... but its not that cut and dry when in actual practice. There's a hole now in your wallet because you were hoping for one thing and didn't get it, and that is a crap feeling. A person feeling angry is a completely normal and understandable reaction, and not something that one can necessarily just "grow up" from, because it applies to adults just as much (hell, more so. Adults are more likely to make bad investments, and many invest a fair bit more that they should.). It's basic human emotion which can easily override logic, even in the best of us, and only a computer would be able to look at something 100% logically.