Things are changing, albeit slowly. It is tourists like yourself who have helped to bring about much of the change, not by boycotting the country but by writing letters of protest and supporting animal charities. I work for a program that feeds and neuters strays which is making huge strides. Apart from the routine work of ensuring the health of thousands of strays, we go into schools as it is the children who will bring about the biggest change. I have had one of my cats shot and another poisoned , but the work goes on.
When I was small, I often spent time at my Grandfather's farm in the country, in the U.K. and saw much casual cruelty towards animals both from the farm hands and the people around. In later years, we acquired two cats, on separate occasions, from different people, both of whom were in the process of being drowned because "they were a nuisance" - the last one was less than 20 years ago. I also successfully engineered a prosecution against a neighbour who was poisoning cats and dogs because they ran into her (unfenced!) garden so it is relatively recently that some in good old Britain are starting to realise that this behaviour is unacceptable.
Re the cats in Greece, they do not breed or they produce much smaller litters when food is scarce.All the kind tourists feeding them at Taverna tables means that they are well fed in the Tourist season and often produce a second litter in September (the first is generally in March). The kittens from the second litters often starve to death as all the tourists have gone. I know many wonderful Greek people who look after colonies of stray cats and dogs but there is a limit to what they can do and many tourist places shut down for the winter. Feeding them is actually a cruelty in itself.
There is cruelty everywhere, education and practical help is the key.