If you're not aware of Crimean demographics, it's never been majority Ukrainian. It was majority Tatar but then they got exiled by Stalin for allegedly colluding with the Nazis. After that it became and remained majority Russian. [1] The reason for the exceptionally high approval is that the demographics made the poll largely a formality - everybody knew it would pass, and so those that opposed secession chose to boycott it, in part on principle, and in part to undermine the poll numbers. But exit polling and various Western polls showed similarly overwhelming support for joining Russia.

As for the US - you could say we had two kerfuffles along these lines, or we'd still be Brits. Win some, lose some. And notably we probably would have also lost against the Brits if not for the French. Granted they didn't then try to take us under their wing as part of France, so the similarities end somewhere - but on the other hand that might have indeed been the exact outcome had we been majority ethnic French. Kind of an amusing parallel timeline to imagine.

[1] - https://www.bbc.com/news/world-europe-26598832