Trademark is specific and you have to register the designs. Rickenbacker have their headstock, the TRC, and elements of the body shape registered.
The companies do have to show they are actively protecting their IP.
It's no different really to fake Levis, Louis Vuitton handbags, Mac clones, Concordski, and so-on.
(For Rickenbacker there is a bit of hypocrisy though, as they are now selling a treble pickup inner cover which is a straight rip-off of a 3rd party one.)