They are FERAL, we can try to manage them with contraception as much as possible but when that becomes ineffective then they must be culled like any other over populated species. Or I suppose we could do what you think the right thing to do and leave them be let them increase in numbers until we have another winter like the one just past, you will see THOUSANDS die from winter kill, have you have ever seen herds of animals starve to death, all, more or less in the same spot? Happens with Deer/Elk here in Idaho.  Now that is sad.