Precautionary landings should be free of charge.
Why?
Because it might just save a life or several
But along the lines of my fictitious (and admittedly somewhat factious!) example, why don't KwikFit waive the costs of fixing poor brakes on cars? Surely it would save lives? It would stop the driver having to worry about the cost and pressing on with their journey when really they ought to get them fixed.
I have to say that I didn't realise the landing fees would be zero for a landing on the basis of a stomach ache or a funny noise from the engine. That's interesting and I'm amazed this system is not abused.