They killed billions on Vulcan in the same movie. I don't think they gave a shit. They had the entire remaining Vulcan population at 10,000, making them an endangered species, when you'd think after a thousand or two years in space, since the Romulans left at least, there'd be more of them off homeworld. Or with enough planetside and orbital ships and personal shuttles to increase that number by a lot.
I'd love it if the Romulans began massive planetary evacuation the moment they began looking to a foreign scientist in Spock to aid their plight, but Abrams & Co. did not think of or present any of that in the movie. But if Nero says they're the last of them, that may be retconned (for idiocy) later, but the presentation is that they're the last of them.