All kinds of stuff gets stolen. I'm not sure why this would be breaking news to someone.
An unlocked/unsecured bike rack on a car is an easy crime of opportunity, much like an unlocked bike. Someone can quickly remove it and walk away unnoticed, and turn around and sell it quickly on CL for a few bucks.
A lock on the rack doesn't totally solve this problem, but it does make the rack a bit less of a target. The number of people walking around with a cordless angle grinder and the willingness to use one to remove a bike rack is considerably lower. If they're busting out the power tools (which make noise and draw attention), they're probably stealing locked bikes and catalytic converters before the go for things like bike racks.