SQLite is in the public domain. It is perfectly legal to create a derivative works from a public domain project and license it however you want. It's not cool and kind of a dick move to put it under a more restrictive license, but it's legal.
It's not a dick move if you are making legitimate improvements -- especially if you still reference the origin. That's literally the idea behind public domain
However that only really works for people who are satisfied with SQLite's public domain licensing. If you are in a jurisdiction that doesn't allow you to dedicate a work to the public domain and are worried about the SQLite developers suing you for infringement at some point, Limbo holds the exact same risk of SQLite suing you.
