While there's only so much privacy you can technically expect from an operator of a web data storage service, I think being upset about an opt-out study - announced only via blog and twitter - that would benefit the operator personally and users almost not at all isn't really unreasonable.

Edit after downvote: I use Pinboard and like it, think Maciej is a fantastic guy, and don't personally have problems with this study, but come on guys. He'll be alright with some dissent.