So first, I think it's true more generally that individuals or organizations have some obligation to fulfil social contracts they make, even when implicit.

I think simply the act of releasing something as open source has a pretty low implication of active maintenance - we've all seen companies "throw code over the wall" and can calibrate our expectations appropriately. However, if the project signals a desire to create a community around it - docs, blog posts, chat rooms, multiple releases, issues in the issue tracker, etc - then I do think that creates an implicit social contract that users will be helped out, and when (as in Steven's hypothetical) there's an apparently active project but participation in that project gets neglected, I think the maintainers are shirking on their obligations.