By my reading, the restriction seems to simply impose some (reasonable?) legal restrictions on contributions rather than ban them out of principle.

Interesting, they've softened their stance. Today, it reads

> In order to keep SQLite in the public domain and ensure that the code does not become contaminated with proprietary or licensed content, the project does not accept patches from people who have not submitted an affidavit dedicating their contribution into the public domain.

But it used to read

> In order to keep SQLite in the public domain and ensure that the code does not become contaminated with proprietary or licensed content, the project does not accept patches from unknown persons.

(I randomly picked a date and found https://web.archive.org/web/20200111071813/https://sqlite.or... )