To clarify, if you feel your money isn't giving you something worth in return, you'll stop supporting them.
If the dev(s) feel(s) the work he/they're doing isn't being compensated well, they'll stop doing it.
That to me is a transaction, there's no written agreement because it's implied by the patreon posts (the deliveries) and your payments to the dev (the purchases, you're buying access to the content that they'll supposedly release).
Like you said, Patreon is indeed a 'donation' platform on writing, but in reality --the way people end up using it on both sides-- seems more like a platform for monthly crowdfunding. Something like a monthly Kickstarter.
reality and seems do not belong in the same sentence, seems to me like not even you believe in what you're saying
the fact of the matter is that patreon is a donation platform, not my fault as an dev that you have some opinions about how you think it should be done, or what else you should get from it.
there are only 2 things you get from donating
1. supporting your favorite/chosen content creator in order for him to continue working on the content you enjoy
2. the bonuses provided by the rank you chose to donate.
there is no contract, there is nothing that obligates him(or any dev) to do what anyone else is saying, it's a donation platform, donate or not, but that's where it ends , with you(general you) choosing to donate or not, other things (dev needs to tell me this cause I pay him!!!, and shit like that , don't fly, doesn't work)
Also you're not buying any content , you're donating money to the creator in order for him to incentivized to continue working on that content.