Robarabian is right on (of course) - LEO's will usually not write for the simple violation of no front plate as it is a red flag that says you can stop me if you want. They stop you so they can take a look and see what they see or smell. If you are cool, they usually let you go after the eyeballing and do not write for the front plate so the next cop can stop you if they want. It's a freebie for them to stop you with the obvious violation in CA.
Now, I have never had any problems with LEO's but I still do not like the idea of them having to pull me over and my wasting their time when they could be out stopping thugs for real crimes. At least that's what cops did once upon a time.
My car will have a fun plate on it just because I do not want to feel embarrassed for not having one and taking up their time. Oh, and having the plate on your dash board is not legal and never was in CA.