I'm still looking for a V.C. to support this but straight of the CA Parks website.
OHMVR Frequently Asked Questions
5)
Q:
A:
I have an Arizona MC plate on my OHV. May I ride it on a California highway?
No. There are a variety of motor vehicles specifically manufactured for operation off of the highways. Off-highway motorcycles, dune buggies, sand rails, and ATVs are common names as well as the vehicles with side-by-side seating for two occupants, commonly called RUV, ROHV, MUV, and UTV. A vehicle designed and manufactured for off-highway use may not be operated on a California highway even though it may be highway legal in its home state because it does not meet the federal highway safety standards for highway vehicles. The California Highway Patrol is responsible for administering the highways in the state.
http://www.parks.ca.gov/?page_id=25733
An Arizona "MC" plate is registration and is accepted for off-highway operation in California. A vehicle with an Arizona "MC" plate may or may not be street legal in California. In order to be street legal, a vehicle must be designed and tested by the manufacturer for highway use consistent with federal regulations (e.g., legally equipped dual-sport motorcycle).
Summary Table
<TABLE style="LINE-HEIGHT: 120%; FONT-SIZE: 11px" border=1 cellSpacing=1 cellPadding=5 width="100%"><TBODY><TR vAlign=top align=center><TD width="33%">
Type of OHV Identification</TD><TD width="34%">
Legal in CA?</TD><TD width="33%">
Legal in AZ?</TD></TR><TR vAlign=top align=left><TD>
AZ RV Plate (title plate) /
No AZ OHV Decal
</TD><TD>
Starting 7/1/09 must purchase CA OHV Nonresident Permit (valid ONLY for OFF-HIGHWAY use)
Note: Operators of vehicles over 1800lbs are NOT able to purchase an AZ OHV Decal so they must purchase a CA OHV Nonresident Permit for OFF-HIGHWAY use in CA.
</TD><TD>
Vehicles under 1800lbs: NOT VALID for ON or OFF-HIGHWAY use. Must have an AZ OHV Decal starting 1/1/09.
Vehicles over 1800lbs: Valid ONLY for OFF-HIGHWAY use in unincorporated areas on dirt roads not maintained by state, city, town, or counties of the state; or roads / trails on federal land that do not require vehicle registration (street legal / licensed). Recreational Use Permit needed if operated on State Trust Land.
</TD></TR><TR vAlign=top align=left><TD>AZ
RV Plate (title plate) /
Valid AZ OHV Decal
</TD><TD>
Valid ONLY for OFF-HIGHWAY use.</TD><TD>
Valid ONLY for OFF-HIGHWAY use in unincorporated areas on dirt roads not maintained by state, city, town, or counties of the state.
Note: Operators of over 1800lbs are NOT able to purchase an AZ OHV Decal. Recreation Use Permit needed to stage (park) on State Trust Land.</TD></TR><TR vAlign=top align=left><TD>
AZ MC Plate (small registration plate). Operator pays full registration fees. No AZ OHV Decal
</TD><TD>
Valid ONLY for OFF-HIGHWAY use. (Exception for Arizona registered motorcycles used primarily on-highway are valid for ON or OFF-HIGHWAY use*)</TD><TD>
Valid for ON or OFF-HIGHWAY use for vehicles not required to have the OHV Decal.** Recreational Use Permit needed if operated on State Trust Land.</TD></TR><TR vAlign=top align=left><TD>
AZ MC Plate (small registration plate). Operator may pay reduced VLT.
Valid OHV Decal
</TD><TD>
Valid ONLY for OFF-HIGHWAY use. (Exception for Arizona registered motorcycles used primarily on-highway are valid for ON and OFF-HIGHWAY use*)</TD><TD>
Valid for ON or OFF-HIGHWAY use.** Note: Operators of vehicles over 1800lbs are NOT able to purchase an AZ OHV Decal. These vehicles will display the full sized license plate. Recreational Use Permit needed to stage (park) on State Trust Land.</TD></TR><TR vAlign=top align=left><TD>
CA Green or Red Identification Sticker (CA resident)
</TD><TD>
Valid ONLY for OFF-HIGHWAY use.</TD><TD>
Valid ONLY for OFF-HIGHWAY use for a maximum of 30 consecutive days.</TD></TR><TR vAlign=top align=left><TD>
</TD><TD>
Valid ONLY for OFF-HIGHWAY use.</TD><TD>
NOT VALID for ON or OFF-HIGHWAY use.</TD></TR></TBODY></TABLE>
The reason they all got tickets with the AZ plates they were driving on a non OHV trail. Essentially it is a non maintained county dirt road meant for Street legal vehicles only.
Not saying I agree with it because I want to ride them too but it wasnt an OHV trail.
DezertTeryx out of curiosity what was the Vehicle Code section you were cited for? If it was 4000(a)(1) C.V.C. that section would not apply to you because you are registered and you could have beaten the ticket. You could have always asked to have court held by written declaration so you dont have to take the day off. Basically you write a declaration showing why the citation is invalid including facts and vehicle code laws. Even once found guilty by written declaration you can appeal it and still show up for your day in court in front of a judge. Sometimes people get cited for incorrect sections and can beat a ticket that way even though they are in the wrong but the officer wrote them for the wrong thing.
Just because a renta cop or LEO doesn't know doesn't make it so! 
It isn't about the money and the more they win the more they think their right! 
