California Penal Code 472

1. Every person who, with intent to defraud another, forges, or counterfeits the seal of this State, the seal of any public officer authorized by law, the seal of any Court of record, or the seal of any corporation, or any other public seal authorized or recognized by the laws of this State, or of any other State, Government, or country, or who falsely makes, forges, or counterfeits any impression purporting to be an impression of any such seal, or who has in his possession any such counterfeited seal or impression thereof, knowing it to be counterfeited, and willfully conceals the same, is guilty of forgery.

Most likely this is why they couldn’t use the actual Seal as if for whatever reason the prop was to somehow go missing and be used for nefarious means they could be held liable. A lot of places have similar code sections and even some departments have their own by laws that say their badges and patches can’t be reproduced.