A poker player can't reverse his decision, but a sportsbettor can - always up until gametime, and often (if a major sport) during the game. You can bet sports without ever having a bet open for the game. A poker player has several rounds to pull out, as does a trader - a trader won't just pull out, there will be reason for doing so, just as with a poker player. Anyway, it's not like there aren't situations where a trader doesn't have the control you suggest.