I am not an expert about this, but...
The CCS1 adapter, like most/all accessories that plug into North American/Korean/Japanese Tesla cars, has a deep notch at the bottom that allows the car's charge port to lock the plug in place during a session (until the owner with his/her fob/key returns).
Yes, someone could, I suppose, smash an adapter out of the charge port with a hammer or something, but the adapter might not be worth stealing after such mistreatment.
So I venture to theorize that the adapter is usually (not always) safe from random theft due to being "locked" inside the charge port during an active session.
A different problem is having someone disconnect the CCS1 cable plug/handle from the other end of the adapter. This would be more about vandalism (petty disruption of charging) or about "stealing" the cable away from one car (before charging completes) in order to charge another nearby car.
The sprung "pin" at the top of the Tesla CCS1 adapter (that helps prevent the CCS1 handle latch from being raised) helps prevent that problem. But I've read that it can't always prevent a charge session from being halted.