I really think that kids are generally more capable and responsible than people assume. I understand that some don't feel comfortable with it for their own kids, but I just don't see a blanket, "Never for any child!" as realistic.
At my DD's school, they have a policy that they only release kindergartners and first graders to a known adult. If someone else is picking up your child, you have to send a note. Kids can't leave until the adult is there to get them. In second grade (age 7 or so), all bets are off. Kids are free to leave on their own. I was just checking kids' fares for our Metra train line, and I happened to notice that it says "Children under age 7 are not permitted to ride alone." So, it appears that many organizations in addition to Disney feel that kids of this age can be given some degree of freedom. Now, I'm not suggesting leaving a seven-year-old home alone while one goes to Mexico for a week, but small and reasonable freedoms work for me. And if Disney says it's okay, then I'll take their word. I'm sure their legal team looked over the "riding alone at 7" rule and gave it the thumbs up.
Most kids really don't need to be supervised at all times. My 10 year old would be horrified if I sat and watched her at all times. She knows the rules, and I trust her to follow them when I'm not around. Even my three-year-old gets some unsupervised time. I put away laundry while she's watching TV, go to the washroom alone, etc. As she proves she's responsible, she gets more freedom as well.