Business logic.
If I start building a project where I use the source code for a FLOSS project rather than just a library then I need to develop with an awareness of two factors: the changes to the code to make it do what I want and those aspects that I would be allowed to release to the world.
Generally it's not that difficult to do this, but if deadlines are tight then I'm not going to 'waste' time stripping out our proprietary extensions.
