Some guitars, for example, various Fender Strats I've tried, stay in tune very well when using the whammy bar. I think it has something to do with the nut and how well it is made. Anywhoo, Ritchie probably stayed in tune when hammering on the whammy bar because he was in the studio at the time and was able do take as many takes on a whammy bar line as he wanted untill it was right. Either that or his guitar was a good one and stayed in tune better.
As for the answer being a floyed rose, that isn't necessarily true. Apart from being very high maintinence, you get what you pay for. Some floyed rose systems are cheaper and don't perform as well as others, they don't come back to float exactly where they did before, they make squeaky noises, etc.