[SQLite]

We are more open to patches on Fossil because (1) Fossil has a BSD license, which is easier to accept patches for than public-domain and (2) Fossil is seen (rightly or wrongly) as less "mission critical" than SQLite.

And, you are right - in many cases people will send a drive-by patch but we will rewrite it rather than apply it directly: to make sure we understand it for long-term maintenance, to clean it up, and to avoid licensing problems.