Response is below:
It's not a law, but a number of rulings by the Society-level Herald's office (aka Laurel Sovereign of Arms). The chain was first protected in the June 1982 LOAR, along with the note in the cover letter: "The question of registering regalia and tokens has come up. As you see, I have now registered them for the SCA-wide orders and titles." It was clarified the following month that the chain was in fact referring to the regalia, the chain around the neck. It further stipulated that "The regalia and tokens of the SCA-wide orders and titles are restricted over the entire SCA.
Since then, numerous Laurel Sovereigns have reiterated this protection. The October 1993 cover letter makes reference to the Board's reaffirmation of Laurel's authority over the registration and use of Society regalia. The October 1998 cover letter attempted to correct the belief that the restriction only applies to heraldry and not to regalia. Finally, in the August 2006 cover letter, Laurel clarifies that any color of chain, not just gold, is protected for Knights, and that pendants hanging from a chain removes it from regalia status.