I don't have the time, nor inclination (nor the knowhow for that matter) to do a legal search, but I seem to remember an article in the AOPA pilot years ago that stated that a LEO had the right to look at your certificate; but that you should hold it because it had been ruled that actually handing it to them constituted "surrender". Of course if you "surrender" your certificate, you don't have the right to get it back.
I don't care what FN thinks, If I ever had any idea that a LEO isn't working in my best interest, I will NOT hand over my certificate. If you think about it, what will they do? For that matter, what would have happened in the ramp check in question? Does the Sheriff have legal authority to arrest a pilot for not possesing a certificate? What if you honestly forgot it? For instance, my certificate stays in my flight case (in the crew room), but I occasionally fly light stuff; it is conceivable that I might fly without my piece of paper. Is that criminal?
In all practicality, we live in a police state, so we should all just go ahead and mail back the certificates today.