They rely on the ignorance and gullibility of the average motorist to scam you out of money. A prime example is your suggestion that they may be able to identify the driver from a CCTV image.
Just to prove the point, go and take a photo, high resolution, of any passing vehicle where you can see the driver clearly. So, you now have a very clear photo of a driver. However, you have no idea who that driver is. Even if you were able to get the Registered Keeper details of the vehicle, you still have no idea who the driver is.
So, how do you propose identifying the driver? Do you know of some magical unicorn database where you can feed in an image and out will spew the identity of that person? Of course there isn't.
The only way they would know the identity of the driver is if the registered keeper blabs it to them, inadvertently or otherwise. Until then, they have no idea who the driver is.
As for "if it doesn't go as planned"... what do you think would happen? This is a well tried and tested response to PCNs where there can be no Keeper liability. Unless they have the identity of the driver, there is absolutely nothing they can do about it.
So, it is not a loophole. It is the law. Personally, I have never paid for an airport pick up or drop off for many years.