It's not cut and dry to differentiate between the act and the wager.

One issue is that prediction markets provide financial incentives to perform actions in the real world. For example, if I want a head of state murdered, I can wager lots of money that they won't be murdered. If somebody wants to earn that money, they can simply bet against me and then murder them.

It's not an dispassionate wager like betting on roulette, it's a wager that directly influences the real world, at least a bit.

Of course you could directly hire an assassin, but that doesn't come with plausible deniability.