Well if some dude went around climbing trees, destroying nests and killing the native birds people would probably complain. The harm humans cause is due to their pets too, cats wouldn't be where they are without humans. When there are too many outdoor cats in an area they breed a lot and effect bird populations which effects other parts of the ecosystem. Idk if it needs to be illegal but if it's discouraged the numbers of cats killing off birds can be reduced. If people can teach their cats not to eat birds then maybe it would be fine.