While most likely not illegal, this feels rather unethical to me. Presumably most of these contests are promoters trying to use the contests as a marketing tool and the prizes as a reward to fans. This bot just aims to take as much as possible.
"OK. 1 more chance. If no one RTs, replies, or even talks about this tweet, I will donate £15,000 to a charity of your choice. Any questions?" --> 369 retweets, most by suspected bots.
I'm not big on competitions or surveys, but I occasionally participate in one spontaneously for amusement, and on those occasions where I've won something it's been highly enjoyable. I'm much less inclined to do if I think about mechanized competitor-bots or suchlike, and think that automating such activities is astonishingly selfish.