[structural]

It allows the code to be fully public domain, so you can use it anywhere, while very strongly discouraging random people from forking it, patching it, etc. Even still, the tests that are most applicable to ensuring that SQLite has been built correctly on a new compiler/architecture/environment are made open source (this is great!) while those that ensure that SQLite has been implemented correctly are proprietary (you only need these if you wanted to extend SQLite's functionality to do something different).

This allows for a business model for the authors to provide contracted support for the product, and keeping SQLite as a product/brand without having to compete with an army of consultants wanting to compete and make money off of their product, startups wanting to fork it, rename it, and sell it to you, etc.

It's pretty smart and has, for a quarter century, resulted in a high quality piece of software that is sustainable to produce and maintain.