On the other side of things, does accepting a PR from someone grant the author the ability to collect royalties for that software project? If so, how would you quantify that? LOC seems like a quick but really bad answer for what it is incentivizing.
For many, the entire point of "F" in "FOSS" is not getting tangled up in this kind of mess. I personally would never want to add "am I opening myself up to a lawsuit if I don't pay someone for their PR" to enter my calculus of accepting a PR. Especially if this is because I reject payment at all on a project.
Finally, if a company/employee submits code to FOSS under this model, do they now expect a return?