I'm not sure if this is entirely true. Does the cat go into shock everytime it rains ?
Of course not, but a human running towards it and then throwing a cup/bucket of water over it hardly compares with rainfall, unless maybe you are in the amazon.
The cat will never understand that doing their business in a garden is a problem - no amount of sitting down and talking to them will get your point across. The only alternative therefore is to have them associate something unpleasant (i.e. the chance of getting wet) with a particular garden resulting in a poop-free zone.
Sure then you might hope to yourself that the cat is in your garden when it starts raining, and they will be so horrified from experiencing such unpleasantness in your garden they will never return. Cats don't really work that way. They are persistant and cheeky devils.
What if I want to have mice in my garden and your cat is constantly scaring them away/killing them ? Mice are as entitled to an existence as cats.