Because that's what they did to the kitten my friend found in the bottom of her bin yesterday. 
They hadn't just dumped it, they actually pushed it right down to the bottom, and covered it with the rubbish - she would probably never have found it if her neighbour's dog hadn't gone nuts, barking and scrabbling at the bin.
Luckily she has been able to find it a good home with one of her family, but I hate to think what would have happened on bin day if the neighbour's dog hadn't been passing by.