It sounds like what they actually did was increase your credit line, not your balance. So if people made trades on the (larger than normal) margin and profited, they can keep the profit. If they took losses, they are responsible for losses, slightly better than altering balances, but only slightly.

It is pretty much the definition of giving your users enough rope to hang themselves with.

It is an INCREDIBLY STUPID move on the part of Zecco. While it could be an accident or bug, I believe that it is too much of a coincidence to be chance.