Twitch recently posted on their blog an update to their terms of service regarding promotions on streams. In summary, the streamer is responsible for all promotions on their stream and must abide by their own local laws as well as claiming all responsibility and liability for the promotions. In addition, you must read out or post this message whenever a promotion/giveaway is run on your channel:

“This is a promotion from [channel name]. Twitch does not sponsor or endorse broadcaster promotions and is not responsible for them.”

Bottom Line: Research your local laws for running giveaways and promotions!