I believe that the prohibition on covering both ears was for that purpose.
BUT!!!
The law was ammended in 2003 (emphasis added, subparagraph d):
[quote]
***Ear Plugs **
[Note: At the close of 2003, the California legislature passed a bill which allows non-custom (i.e., foam) earplugs to be worn by motorcyclists. The updated version of the relevant code is posted here.] *
For those concerned about hearing damage, and who therefore wear earplugs when riding, here is the law as it pertains to you.
[ul][li]*27400. A person operating a motor vehicle or bicycle may not wear a headset covering, or earplugs in, both ears. This prohibition does not apply to any of the following:[/li]li A person operating authorized emergency vehicles, as defined in Section 165.[/li]li A person engaged in the operation of either special construction equipment or equipment for use in the maintenance of any highway.[/li]li A person engaged in the operation of refuse collection equipment who is wearing a safety headset or safety earplugs.[/li][li](d) A person wearing personal hearing protectors in the form of earplugs or molds that are specifically designed to attenuate injurious noise levels. The plugs or molds shall be designed in a manner so as to not inhibit the wearer’s ability to hear a siren or horn from an emergency vehicle or a horn from another motor vehicle.[/li]li A person using a prosthetic device that aids the hard of hearing.[/ul][/li][/quote]
Unlike TV Time, I olny wear earplugs on longer trips (100 miles or more). When I do, I use the little yellow cylindrical plugs. Are they legal? As long as they ‘do not inhibit the wearer’s ability to hear a siren or horn’. Seems as if there’s some leeway there. The yellow plugs are not designed in such a manner. I can hold a conversation while wearing them. But I can also jam them in so tight that I can’t hear a thing.
A most civilised thing, indeed! 
From here: