>On prediction markets traders can bet both sides

In fact traders have to be there for both sides. The article means approximately that on 1c bets on unlikely things the people betting 1c to get a dollar if it happens do worse than those betting 99c to make a dollar if it doesn't happen.

I'm not sure that means betting 99c to make a dollar is a great business though - your money is tied up, often the volume is low so if you can only bet say $99 to win $100 it may not be worth the hassle to make $1, and you are vulnerable to the bettors knowing something you don't - maybe the unlikely event isn't really that unlikely but you don't know.