Originally Posted by PersonaNonGrata

I don't think they are legal if you follow the Vehicle Code strictly.

CA Vehicle Code Section 5201:

"(2) The installation of a license plate security cover is not a
violation of this subdivision if the device does not obstruct or
impair the recognition of the license plate information, including,
but not limited to, the issuing state, license plate number, and
registration tabs, and the cover is limited to the area directly over
the top of the registration tabs. No portion of a license plate
security cover shall rest over the license plate number.
(g) No casing, shield, frame, border, or other device that
obstructs or impairs the reading or recognition of a license plate by
a remote emission sensing device, as specified in Sections 44081 and
44081.6 of the Health and Safety Code, shall be installed on, or
affixed to, a vehicle."