You see it’s not a coinjoin ban. Because op_return is not used in coinjoin tx but rather in tx0(transaction zero), which is not a coinjoin txn! 
Also Tx0 fees are paid to the software publisher, not to the coordinator and no fee is paid during mixing, except fees that paid to miners. then tx goes to premix/postmix which belongs to your own derivation path. 
Therefore op_return contains info allowing the server to verify that the fee was actually paid to an address., because sending to whirlpool means sending to your own hardened derivation path that you control. It's an anti-spoofing mechanism. If the fee is not seen in the blockchain then the inputs are not registered. It also allows to not use a static fee for address collection.
The use of op return in tx0 resilient to potential coordinator failure and enable decentralization - two things a coordinator database can't solve. 

 Thanks for clarifying