Here's an edited version of a puzzle from the book "Chuck Klosterman four" by Chuck Klosterman.
It is 1933. Somehow you find yourself in a position where you can effortlessly steal Adolf Hitler's wallet. The theft will not effect his rise to power, the nature of WW2, or the Holocaust. There is no important identification in the wallet, but the act will cost Hitler forty dollars and completely ruin his evening. You don't need the money. The odds that you will be caught committing the crime are negligible. Do you do it?
When should you punish someone for a crime they will commit in the future? Discuss.
Only if (a) people know you did it, and why; and (b) you're not a one-shot time traveller, so that there is the potential for this kind of pre-punishment to happen again.
It can still be effective if they don't as I discuss here.
[Insert standard TDT argument about how by doing this, you're acausally increasing the number of other time traveling pre-punishers.]
However, your main point, that the effectiveness of this scales non-linearly with the number of punishers is correct. However, this appears to be more of an acausal co-ordination problem.