Social norms vs letter of the license

I think you need to account for intent as well. If I do something for myself that I think possibly someone else might want to incorporate in something they are doing, I'm going to license it very permissively. But if I were to start working on a project that I wanted to work on collaboratively for an audience of people, I'm going to probably use a copyleft license to make it harder to capture by an outside party.