Rather than viewing it as a predictive system, I think it's more pragmatic to view it as a kind of poll. Basically it's a quick and dirty way to check general sentiment around certain topics.
I don't know about that one. The people that it 'polls' aren't that representative, leading to heavy bias. Which is fine, you can correct for that. It's just that I don't know their biases!
It's a marketplace which is illegal to bet on in the USA, and commonly creates betting markets for US political events. What an interesting self-selection of users that creates.