I don't mean "okay" in a legal sense, but rather a moral sense.
It reminds me quite a lot of http://i.imgur.com/5quY1Iq.png except that the difference is that Maciej is a researcher and isn't disclosing the data to people. However, he's still going through people's private stuff. Notifying them that you're planning to go through their private stuff is the most basic common courtesy; it's why landlords can't simply walk in to a tenant's house whenever they feel like it, even though they own the property.
Let's put it another way: I didn't know Maciej was the type of person to trawl through people's private information that they trusted him with. If I did, I would've investigated other options for a bookmarking site a couple years ago, or would've written my own, and I wouldn't have breathlessly recommended Pinboard to whoever would listen. The recommendation would be more like "Pinboard is great, but the owner likes to look at your stuff, even if it's marked 'private,' so keep that in mind."