I can't speak for OP but here goes as an adult who sails with
DCL both with and without children. As a matter of fact, my next two cruises are sans teen. We sail DCL because we enjoy the product for both adults AND children.
1) I expect children in their areas. This is most of the ship and if I crash the Vibe or Edge I would be expected to leave...not that I could crash since they are card controlled, but hypothetically.
2) I expect parents to parent which includes respecting the rights of others, following the rules and reading signs-aka situational awareness.
If this doesn't happen, such as the time there was a 6 year old gleefully playing in the Rainforest Room, you can bet that I will be The One to report it to staff and expect a resolution. Sometimes age limits (either way) are for safety, sometimes they are for liability, sometimes they are just the way it is. If you go into an adults only area and expect there to be no children, I don't understand how that's the adults' fault if they are displeased that there are children.
I disagree with your analogy. To me, it's like disregarding the airport's posted signs that say "no trespassing," walking near the runway and complaining about the noise.