Every GP bike has had the cassette gearbox since the early 80s. What the AMA rules state, is that you aren't allowed to change the stock ratios, not that you can't race a bike with that capability (although any bike can have the internal ratios changed if you're willing to pay the freight). I guess if the AMA Superbike series were the only series on the planet, this would be a waste of engineering/time/money/development etc., but since they are only one national series among 12 or so, and we haven't seen the same restrictions in WSB or World Endurance then it may be an excellent move on Honda's part.
By bringing this type of system to sportbikes, you can bet that Yamaha, Suzuki and Kawasaki will have similar systems very shortly if it proves to be of value.