I understand the need to sign up, but where do we go from there, and does it matter if you can't afford to donate?
Chances are, they'll likely send you an email confirmation with a link to click to make sure it's you requesting site access.
As to donations, if it's an honest group or organization, that's what it'll be, is a voluntary gift of cash, sources to other materials, whatever, from those who can donate. They'll let all users have equal access.
That's how donations are supposed to work, there's links all over the place for this, even some of the browser extensions asks for one, as well as Wikipedia (one that I donate $10 to yearly). That's not much, but probably more than 90% of it's users gives.
There are some sites though, that's selling products behind a donation mask, such as the "premium" editions of Zorin OS. It could be for two reasons, or both. First off, a donation isn't a sale, so that site doesn't pay income tax on any collected cash. Secondly, and the one that impacts it's consumers the most, is they're getting no priority support for these editions, just forum support like the rest, with an added email access for support, but no guarantee of a 100% correct answer in a pinch, and most certainly no "on site" support. That's worthless to one with a Professional edition using the OS for business with customers waiting & the computers are down, and for those with Ultimate edition looking to finish the setup a killer home theater to throw a huge party tonight. If I'm paying for an OS, I expect priority support 7 days per week, not when some dev feels like contacting me.
Zorin probably isn't doing anything illegal in their region, doing just enough to stay inbounds of the law. If it were a US based corporation, LLC, or charity, I'm sure the IRS would be looking into the donation aspect of their affairs, and likely wouldn't exist (lor long). Of course they're not the only offenders, but the first that comes to mind.
I say go through the registration process of the Open Library & see how it turns out, may be a great wealth of Linux offerings at no cost.
They have over 500 Linux e-books, so it's worth a shot. The part I don't understand is the "waiting list", seems that with an e-book, this wouldn't be necessary. But nothing wrong with at least going through the procedure & seeing firsthand how it works. If a donation is required for priority access, then we'll know what the deal is. I'm signing up myself to see if there are any catches.