Those predictions are just market data, market data is often very expensive (my data usage at work is probably about $100k a year and i'm not even a trader), I imagine polymarket will be too when they are more established.
It's inherently free data. You have to be able to trade against it, and to trade against it you need to see it _before_ you give them money, so they can basically never charge for it. When they sell it to people, it's just laziness, they could scrape it themselves.
edit: apparently they do, that's even more wild lol
Traders sometimes go to prison for insider trading. The wisdom of the crowd human when the crowd is following someone with insider information is deemed legal.
