Ownerless cats may look the same, but there is a difference between a stray and feral cat. A feral cat is one that is born and raised in the wild with little or no human contact. A stray cat is one that has been abandoned or left from home and became lost. A stray cat is homeless but has already been habituated to humans.
There is no question that when cats scratch objects or even people it is not pleasant. But scratching is a normal behavior for cats and serves many purposes. Scratching serves to shorten and condition the claws, and more importantly, cats scratch to mark their territory.