Innocent until proven guilty is great for criminal court.
Protecting users against extraordinary claims which have onus (the claims must be proven) is great for the forum.
You would rather temporarily cease transactions to stop a likely scam than wait for the scam to occur, then act upon it.

In both scenarios, one must disprove the null hypothesis. However, the difference is that in the former, the null hypothesis is guilt and in the latter, the null hypothesis is innocence.