The Cat-Crap Chronicles and 'What's that supposed to mean...?'

WARNING ! PLENTY OF SCATALOGICAL REFERENCES IF YOU ARE SQUEAMISH STOP READING!
This morning the 'Tiny Hands' and I watched in a horrified breakfast stupor as one of our neighbours cats curled up a massive one and wrecked our beetroot seedlings by covering it up. As I had a day off from the Hall of Plenty and being the very bitter gardener that lost a lot of edible crops last year to cat pooh, I decided to take affirmative action. I wrote a note that I posted through every letterbox on both sides of our alleyway to the cats owner. The cats owner has just gripped me in spectacular fashion by my compost heap,I think I may have caught her coming out to clean up the pooh I had flung into the alley. Anyway she waved my letter at me and demanded,
'Did you write this.'
'Yeah,' I said.
'Well what is this supposed to mean?' She pointed to a sentence in the letter.

"....and so when I discover which cat belongs to whom, the owner will receive it's cats crap back in a lovely plastic bag on their doorstep."

Now, I am pretty sure that is about as transparent as a sentence can get. So when I told her, she called me,' arrogant'
When I said, 'Tell me something I don't know!' She called me a 'cat hater'
Which I think is absolutely out of order, anyone who knows me knows that I greatly admire anything so arrogant, independent of spirit, cruel, hunts for pleasure and that plays with it's prey before killing it.I simply dislike the wholesale crapping on my raised beds.

Which brings me to my point since when has, 'What's that supposed to mean?', been such a conversation get out clause. Recently everyone has been saying it. The cat owner, customers as you tell them that their skin is actually dry not oily and the woman on the shop floor who was having an affair for 18 months, then moaned about her husband getting suspicious. When a colleague said,
'Don't you think he has cause to be, you have been shagging someone else!'
Said..... and I quote,
'WHAT IS THAT SUPPOSED TO MEAN?'

If you are already stating the bleeding obvious what else needs interpretation? Is it a statement people make while their brains absorb the
information. Is it something they randomly say because they can't quite believe anyone would say anything so obvious to them, or are they really that stupid?

It means what it say's and it doesn't mean anything else!