Same market that was 90:1 that Obamacare would be overturned by USSC. Yep, they’re infallible.
Predictions about a Supreme Court ruling bear no resemblance to predictions about election results. Markets like this work by "pricing in" information relevant to the prediction. The Supreme Court is, effectively, a black box: until a decision is released, there is very little information with which to make a prediction. With an election, on the other hand, there is an almost endless stream of information one can use to: polls (even skewed polls provide useful information, as long as the internals are released), economic data, other external indicators, etc.
Those gamblers at Intrade do not have any information beside media polls to place their bets... absolutely nothing else... So they follow the polls and make their bets... If they are too stupid to bet based on super biased polls I hope that many of them lose a lot of money...