A disclosed affiliate link is a checkable claim about the author. A hidden one is a claim the author hoped nobody would check. That difference is the whole mechanism, and it is the same one the proof you make runs on: a claim is something you say, a proof is something you made, and the strongest proof is the fact that would embarrass you if it weren't true. "I get paid if you buy this" is that kind of fact. It costs the affiliate something to say, which is exactly why it works as a trust signal and a hidden link doesn't.
It is also not optional, which helps. In the United States the FTC tells affiliates to disclose the relationship clearly and conspicuously, next to the link. In New Zealand the Advertising Standards Code says advertisements must be identified as such. Neither rule is the reason, though. I don't really want this to be ethical, I just want it to be rational, and the rational case is simple enough: a reader who finds a hidden link has to decide why you hid it, and none of the answers survives the years clock.
What the links do
Every tool link on these sites goes straight to the tool. There is no page of mine in between, so the address you land on is the maker's. The disclosure sits where the link does: a line on every page that carries one, a tag on each tool I list because I like it and that pays me nothing, and on the tools pages, what I get next to each one that pays. How the whole arrangement works, and what I earn from it, is on the links page.
Every category carries more than one tool, so there is no incentive for me to push one over the other. The income per tool is small, and which one you end up with is not a big thing to me. That isn't modesty, it's design. A list of only the tools that pay me would be an advertisement wearing a list's clothes, and the ones that pay nothing are what make the rest believable.
The same rule applies to advice. Most practitioners in this industry have something to sell you, and so do I, a book, so it is fair to assume my advice bends toward it the way everyone's bends toward what they sell. Saying so doesn't remove the bend. It just puts it where you can see it and correct for it.
The test
The test for any tactic is whether it still works once everyone knows how it works. If a tactic relies on the customer not knowing about it, it's manipulation. A disclosed affiliate link passes that test, and a content farm doesn't. Naming the technique before using it does not stop it working, which is the uncomfortable part, and the reason to say so out loud. These sites run that test in public: the trust algorithm at full transparency, to see whether it still works when everyone can see how it works.
This site has no tools page of its own, and that is deliberate. The theory layer carries no tactics, so the shopping lives on the sites that teach the work. What this page carries is the argument for why the other sites can publish their tools without it costing them anything.