That was the hardest thing to decide on.
The way I see it, it's harder to beat 50 bowlers than it is to beat 15, so you should get just reward for your effort. Don't forget, it's your highest 5 point-earning events that will count towards your tally.
Having said that though, in your bigger events, Salisbury Singles and Playford Open, not all the bowlers will be members of the BBA SA Scratch Series, you will only gain points for each person you defeat THAT IS A MEMBER (ie. 80 bowlers may show, but 35 are registered for the series, the highest placed member receives the 35 points). The tournament winner can't just come along after the win and decide they want to participate in the pointscore, they must be a member of the series prior to accumulating the points on the day. That means the $15 membership fee can be paid on the day, but before the event starts.
Even if one bowler wins both Salisbury and Elizabeth and then decides not to bowl any other events (which is unlikely in the first instance), I doubt very much that 80 points (40 points per event, if we get that many members) will be enough to win the end of year prize. However, not bowling these two events (double points) could be to your detriment.