I think people have either forgotten that it was a child who took them, or maybe got diverted into a theoretical debate.
If the latter then I agree, but not for a young teenager. They make all sorts of silly decisions at that age and don't have the common sense experience and being a grown up generally instills. She did a silly thing and I would imagine had her own erroneous train of thought about it, and is now rectifying her mistake. Hotel not out of pocket, young adolescent learns a lesson, so all is well with the world.
For adults, it's rather different im afraid and I do think there is a lot of thieving and pocketing of anything left in public places or '3rd places' which aren't home but aren't totally public either, like cafes, aeroplanes, airport lounges, and hotels.
But far from clapping them all in irons, I'd use more positive ways of directing people's behaviour, it works better and is more appropriate for the transactional relational a business and customer have, assuming the business wants to give a positive experience which leaves customers happy and themselves not out of pocket. Like a polite note, or a set of guidelines for the hotel stay, or doing a housekeeping check before check out as many hotels do already.
I used to travel a lot for work and stayed in hotels all over the world, from basic to super premium, and it generally works better to set the tone for the way you want people to behave, rather than leave it up to people's consciences and experience of travel / hospitality. If they show they expect good behaviour off guests by treating guests well and setting a positive and transparent tone, that's what you get (mostly!).
The only hotel I've ever been to which treated the guests like potential criminals was one in Ibiza which tbh had a point - it was on the club circuit & had a list of fees for such damage as 'mattress in the swimming pool', 'urinating in the corridors'. Bloody hell!