Why do you think they should disclose it? (I'm actually curious). The only users affected are those using their own affiliate links so it would have no effect on the vast majority of users.
It seems to me that companies are being asked to disclose more and more these days even when it doesn't really affect users and the company has no reason to disclose the information.
It could definitely piss off users who have their own affiliate links [someone upthread said the service they're using won't remove existing affiliate links, btw.] That's a small number of users to be sure, but they may be important users: consider people who run taste-making blogs that make money via affiliate links. If a number of popular blogs like this realized their affiliate links were being removed, and complained about it, it could make for some pretty bad PR for pinterest, and it would be well-targeted bad PR, since that's exactly the community pinterest appeals to.
I think leaving this undisclosed would be a bad idea, as the rumor mill could start up, and spread misinformation about what's happening with affiliate links. It would also be a good opportunity to get feedback on policy.
I think it's smart to have it disclosed somewhere (FAQ) because people that know what affiliate links look like can figure it out themselves anyway by looking at the links.
I don't think there is an effective way to hide the use of affiliate links, so they may as well disclose it in their FAQ or TOS just to avoid complaints that they're not being genuine towards their users.
I don't think they should hide it but I still see no reason to disclose that. I sort of understand your position but I don't understand how not disclosing something that doesn't affect users is disingenuous.
Sure there is, if you are talking about the links on the pinterest items users hover over and not the eventual destination url of the merchant. Just use an in house link shortner that does a lookup to the original destination url, identify if an affiliate program exists, modify if needed and complete the redirect.
Unless you use an iframe, the affiliated URL will be in the user's address bar at the end of the exercise either way. If you do try to trick that away, there'll still be an HTTP request to the URL unless you proxy the entire page. Someone interested or sufficiently bored will be able to see what is going on.
"Effective" masking of links is relative. To some members of the HN community saying that it is impossible to mask the links is the only empirically correct statement. But my aunt and her pinned collection of knitting crap isn't going to break out an HTTP sniffer to see what is going on behind the scenes, the masking of overt monetization so users continue to labor under the delusion that they aren't being marketed to could be all that is needed to be "effective".
Camouflage doesn't render a soldier invisible but that doesn't mean it's not effective.
It'll only be effective until a well-placed, shrill-toned media story exposes it to everyone. Be upfront about it and you'll be less likely to find yourself on the wrong end of "deception" charges, however trifle and ultimately meaningless the deception is.
I can think of a couple of ways that might (!) work to hide affiliate links:
1. Add a click handler to your anchor tags that redirects to an affiliate-linked version of the href.
2. Modify the url on mousedown (since it happens before click.) I think someone told me this doesn't actually work, but I don't remember for sure.
It seems to me that companies are being asked to disclose more and more these days even when it doesn't really affect users and the company has no reason to disclose the information.