there is no "grey area" in california anymore. i've researched everything there is to know about the laws, posted them numerous times, and if anyone from california who's on this forum can't figure it out, it's because they don't want to, and they want to believe they can outsmart the law.
a motorized bike can be impounded for many reasons.
-not having it registered and not displaying the plate.
-not having an M1 (motorcycle) or M2 (scooter and moped) classification on your license.
-not having a license at all, or a suspended license
-riding while intoxicated or committing a crime while on the bike, or using it to get away.
-having a warrant and being taken into custody. (this one, like a car, is up to the officer's discretion. he may allow you to park the bike, or have someone else pick it up. unless any of the other things i mentioned apply.)
he CAN'T (legally) take your bike away for equipment violations, as long as it's registered, and you have an M1 or 2 license. then it's just a fix-it ticket. (and we DON'T need turn signals or insurance.)
if you wanna be some kinda hardcore biker and not wear a helmet, not have a license, and not register your bike, the only person to blame when you get caught, get fined, possibly arrested, and get your bike impounded, is yourself.