Been reading a bit more, and it seems like it's implemented at the user interface level and not the blockchain level as far as I understand. That's why they could do it so fast.
And that's why openledger doesn't have it, you're sort of paying for using the wallet.bitshares UI instead of another one.

The reason they introduced the fee was to make a pool that can sustain the smart coin prices in crazy dips like these when bts is plummeting. So the new fees collected would be used to create more bitCNY and bitUSD because many of them are disappearing due to margin calls on both.

I still think it's more valuable to have a fee-less exchange, but this is what the committee has decided for now.