It's a belief, not a soft rule. So it isn't in the Jurisdiction of GMs to "ban" 10% betting rule.
Who wants to fuke for 20k and get 100TC as a counter? If you win, no gain, if you lose, you lost to a super small counter that isn't even worth anyways. 10% and so on is a good, balanced rule for betting servers. Without it, most high stake bets in some popular servers would ever exist.