Amazon is really hands-off with acquisitions for the most part such as Zappos and Audible. They primarily add to the offerings by cheaper fulfillment or server/bandwidth/cpu costs and more. I think they will treat twitch better than if it was just integrated into youtube.
Also, it shows a dedication to game developers and supporting marketing of games on their platforms and others. Amazon buying them is actually better for Twitch and the gaming/let's play type new marketing channels.
I wouldn't even be surprised if down the road they'd announce a Steam-like digital distribution thingymagick. Steam and it's competitors are rolling in cash and have been getting quite some flack recently so now might be the best time.
EA has the only competitor that comes close, but their tarnished reputation in the gaming community has sullied the only worthwhile adversary Steam has.
2) As Amazon enters the online advertising space to compete with Google Adwords and Adsense, they'll want to own web properties with high impressions for their display ads. User based video creation is great for that but comes with risk for copyright violations. Twitch solves both these issues as it'll give display ads high impressions without much concern about copyright violations as these will mostly be legit user-originated content.