Some people steal. It happens. I've had money stolen from hotel rooms before. For the biggest theft, it was necessary for the thief to extract the cash from inside a zipped bag left underneath clothing in a drawer. This obviously wasn't a crime of opportunity; whoever did it was actively looking for cash or valuables. (This was not a Disney resort, by the way).
Every Disney resort I've been in (with the exception of the studios at Old Key West, and they've started to put them in those rooms) have a room safe. It's provided to hold valuables that shouldn't be left out where they might be accessible. At the very least, the housekeepers have daily access to your room so there is at least one person coming in who could possibly steal whatever's left out in the open (and by this I also mean zipped up into a purse or suitcase... anything that isn't locked). There are other hotel employees who have access to rooms.
If you have cash or anything valuable that you care about, don't leave it in a hotel room. Take it with you or lock it in a room safe. Hotels are legally correct in stating they are not responsible for items left in a room, because hotel rooms are by definition accessible to people other than those paying to stay in it. Nobody's guaranteed the honesty of those people who might access the room. If they did, they'd have to do extensive background checks of all hotel employees.
If you're in the room, always throw the bolt and use the bar lock if you want to be more certain no one can get in from outside. If you're not, use the room safe for anything you care about losing. If you have an item that is too big, see if the resort has a main safe at the front desk (I've used one at the Beach Club).
Someone (was it the OP)? wrote about paying money and being entitled to assume safety. Where other people have any access at all to your room and belongings, you can never totally guarantee safety. Take the available precautions.
Oh, and if it does happen to you, by all means report the theft. It's a crime.