I can't speak specifically to PAF and indexing, but I enquired about this a while back for PAF for Mac (Family Records) and was told that they had licensed code from third parties, so therefore not only could they not open source it, but they couldn't give it away for free either due to royalty requirements.
Code that they've written entirely in-house wouldn't have those restrictions, and I do know there are some projects they would like to make more open, but they are cautious and must make sure everything is done properly, so it can take a while. There are legal issues, perception issues, the support and infrastructure needed to maintain the projects and community over time, and sometimes technical issues of cleaning up the code and removing any non-public information from it.
They do have a somewhat open source-ish project in the works now for the ward/stake web sites. They will probably work on the kinks there and then start opening up more code as they feel comfortable.