What if they had a simple policy of contacting the customer if possible in these situations and left it up to the customer to decide if the property was valuable or not? Personally, I see a difference between something scribbled on a napkin and a set of headphones.

My guess is that the issue is less one of legality and one of effort. I don't know how often customers leave items behind, but I would suspect that it may tend to happen more than we think. As such, at a large hotel it could end up being someone full time job to do nothing but look up customer contact info and try to contact them when something is found. It is just easier and less costly to wait and let the customer contact the hotel should the item(s) be valuable enough.