Donations is just another model of making money from software. So you're going to use App Store for free and make money without Apple getting their cut. It's understandable why Apple does not accept it.
It's not free. There's a $99 membership fee every year, even for an altruistic person offering nothing but a single useful extension/app that makes zero income.
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Let's imagine a feature request on GitHub:
USER: Safari supports the WebExtension API now. Please port your cool extension to Safari.
DEVELOPER: Cool. I'll maybe try doing that this weekend!
USER: Oh, by the way, you'll need to be on the hook for $99/yr for the rest of your life, so that Safari users can install the extension.
DEVELOPER: <...RADIO SILENCE...>
You can complain that it's not right to charge for the program, but the developer program has a cover charge to keep out the riff raff. I don't think this is a bad thing. We all know how much junk is in the App Store as it is, and how long it takes to get a review. Imagine if they didn't charge for the opportunity to submit apps? How much soon-abandoned junk would suddenly appear, and how long would it take for proper apps to get approved?
You're going to have plenty time to Wrap Your Head Around this dynamic by simply observing how the Safari WebExtension ecosystem plays out.
> the $100/yr for the developer program is not a lot of money. [...] It's less than a single restaurant lunch per month.
Is that $100 for a meal for a single person? Where do you eat and how much do you spend on food a month?
Wow you must be superhuman. I misunderstood you. Now I get it.
I don't understand it, so I don't think it's fair to call it "understandable".