I think you are talking about network fee for placing an order, while I'm talking about both network fee for placing order and market fee for a fill.
CCEDK OBITS/BTS Market
Centralized exchange will charge X% market fee from both the buyer and seller in the receiving asset. It looks like CCEDK has their market fee configured to 0.2%. So, if someone sells 100 OBITS, seller pays 4 BTS to CCEDK, and buyer pays 0.2 OBITS to CCEDK. If someone sells 10000 OBITS, seller pays 400 BTS to CCEDK and buyer pays 20 OBITS to CCEDK.
Openledger OBITS/BTS Market
Let's say that issuer of OBITS configures 0.2% market fee on Openledger. Let's also say that core exchange rate is set to 20BTS/OBITS. If someone sells 100 OBITS, seller pays "10BTS worth of OBITS" as network fee (that is 0.5 OBITS) when placing the order and no fee for a fill. Buyer pays "10BTS as network fee" when placing the order and on top of that, they will also pay 0.2 OBITS to the UIA issuer when order gets filled.
So, buyer of UIA pays two times, once when placing order and again when receiving the UIA.
Don't you think UIA seller should also pay two times, once when placing order and again when receiving BTS?
In the current system, the UIA issuer can only charge market fee to the buyer, but can only receive "10 BTS worth of OBITS" from the seller, and that is the limit.
On the centralized exchange, there is no limit because transaction fee is based on buyer's and seller's traded volume.
That means, centralized exchange has no incentives to do business with BitShares because they can collect more money if they use their own proprietary system.