After they're inside Scrooge's mind, the Beagle Boys have trouble differentiating dreams from reality -- again, exactly like the characters from Inception, who need special items, or "totems" in order to tell dream from reality.
When Donald Duck enters Scrooge's dream to help, he has to figure out a way to pry the Beagle Boys out of there. In Inception they use "kicks" to make controlled exits, like how the feeling of falling usually snaps you out of the dream. In McDuck's head, they do, well, the exact same thing.
In order to escape danger, Scrooge starts jumping from dream to dream, but like Cillian Murphy's character, he can't remember the last dream once he's in the new one. Donald, however, is the invader (like DiCaprio) so he alone is able to remember the progression.
But Inception was a sci-fi thriller, and that was key to its appeal: It took metaphysical concepts, like Lucid Dreaming, and used them to fuel the action. The characters can't break the rules, but they can "imagine a bigger gun," and that's hardly something you'd see in some Disney duck carto-