I have to agree with you, wagering race should be simple. Every bet should be counted, without any need to opt in unless there is requirement behind the prize such as wagering requirement.
As far as I can see here, the games are also limited so only bets on the selected games are counted to earn points.
Maybe this is the reason why there is an opt in button in order to participate, because there are min bets and limited games to play.
But still, the terms they have is a bit strange if we compare it to other common wagering race.