The library author has already invested probably hundreds of hours creating the library; if someone who benefits from it only has to spend tens of hours writing the unit tests, they've saved an order of magnitude of time.
We need to stop thinking of OSS as something we're entitled to, that if a stranger doesn't put in enough free labour we're allowed to complain about it. I try to view my dependencies as favours other people have done for me. You wouldn't nitpick about a lack of tests after someone wrote a whole charting library for you; you'd thank them and add your own tests.
And we need to stop thinking of criticism as complaining. I'm very grateful for the OSS dependencies that I use in my projects and often contribute back to them, but I wouldn't touch a project without tests with a ten foot pole. I don't mean that in a bad way, for others it might fit their requirements fine. I'm guessing the author of this project would like people to actually use it, so I don't know why we should avoid giving this sort of feedback just because it's free labour.
Yeah, it's fair to provide criticism... my issue was more with the idea that the library author is responsible for doing what the consumers request, that the burden is 100% on the author. In my ideal world, this project would now be a community effort, and anything the community wants can be implemented by the community.
We need to stop thinking of OSS as something we're entitled to, that if a stranger doesn't put in enough free labour we're allowed to complain about it. I try to view my dependencies as favours other people have done for me. You wouldn't nitpick about a lack of tests after someone wrote a whole charting library for you; you'd thank them and add your own tests.