Well, obviously they're reserving the right to enter cabins to rummage for vibranium, because, duh, it's already a known fact that it cannot be detected by metal detectors.
On a bit of a more serious note, I think we're over analyzing it. If there was more to it than clarifying a policy that's always been in place, they would've just removed the DND signs all together. Why have them when they could've just sneaked the clause in the fine print of the Cruise Contract? I don't know...I could be wrong. Things change. Times change. Travel in general has changed in the last 20 years. TSA, liquids, border crossing requirements, etc. Disney has plenty of their own rules and policies they don't enforce properly, especially in the parks. Would I like it if someone barged in on me? No, no more than kids running in the hallways like it's a stampede, but considering how lax Disney is about rules, I for one am glad
DCL isn't anarchy. Is it an inconvenience? Of course. Does it have potential for screw ups, and violations? Uh, yeah. Everything and everyone does. If you haven't heard of the latest kerfuffle involving United, Google it. At least DCL and Disney in general isn't Level United yet, right? LOL.