Apparently it's 'none of my business' 
I shouted at four people in five minutes today. I did try saying politely 'oh you've dropped something' and only shouted when they said 'I don't want it anymore'.
I so wish I could vomit at will so I could chuck up on them because 'I don't want my lunch anymore'. 
Where do people get this idea that litter is someone else's problem? 