There's the fundamental issue that, if you send a patch to a mailing list, it's hard to tell whether the project has a right to distribute it under the project's usual license. This is somewhat less of an issue if the project uses a strong copyleft license, which you'd be violating if you distributed your patch without licensing it to the recipients. But someone could legally take a BSD-licensed project, change it, and distribute the result under a "only redistribute verbatim" license. And when someone's diffs as posted never include anything about licensing (since they're patches to the middles of files, with all of the license stuff outside of the context), it becomes hard to legally justify redistributing the work without some sort of additional statement. (It's even plausible that someone might want to make a post like "Please don't change free function X like you propose, because that breaks my all-rights-reserved use of the function, which I will reveal to illustrate my argument, but which I'm not contributing.") So it makes sense to ask for an agreement with contributors about licensing their work, even if it is a minimal "I license my contributions to the project under the terms the project licenses their versions to me under."