They didn't need time or funds to put something outside the catflap opening to prevent access.
I know this will probably not hold any water with you, but most people who don't have cats don't really give cats a second thought, unless they're pooing on their garden. I'm sure they'll now learn the lesson the hard way, but contrary to what a lot of cat owners seem to think (especially on this thread), their cat is not the centre of everybody's world.
If they'd left a window open and a human had got in and damaged their rug, they would been seen to have been negligent.
Really? If an adult got in through an open window, it wouldn't be a police matter? And if a child got in, you don't think the police or social services might want to speak to the parents?
At any rate, a human could get in by opening an unlocked door, so by that reasoning, you could have gone into the back garden to hang out some washing, somebody lets themselves in your front door and trashes your kitchen, and it would all be your own fault for not living in a fortress?
I remember reading ages ago about a burglar in the USA who got in to somebody's garage (they were on holiday) and then couldn't get out again (much worse for wear) until they came back a week later. He successfully sued the owners for 'wrongful imprisonment' and won compensation, but I never thought I'd hear a non-American person (assuming you aren't; apologies if you are) defending this kind of thing as 'justice'.