I agree the more available coins on bitshares the better, no doubt about it!
The bts blockchain charges a fee every time there's a transaction, that doesn't go to the gateways it goes back to the bitshares reserve pool.

But if you trade a token that's not native to the blockchain like open.btc or bridge.btc then the company who created the token can choose to take an additional market fee, usually 0.2%. But if you trade on openledger directly you don't pay that fee, just the 0.00092bts for the transaction which is nothing.

Rudex and openledger also earn money from being bitshares witnesses, and have other side projects, so I imagine cryptobridge is doing something similar to maximize earnings.