BCNEX gave bounty hunters an option to exchange BCNX tokens earned from bounty for XRP. Now, you cant get BCNX back. You can convert it back. I think thats BCNEX team decision to prevent dumping once listed.
I guess its way to prevent dumping as you said which is a smart decision from the team. So the token doesnt get devalued.
it is because the bcnex really want to make their coin more valuable, to make sure that the price get high and as you said, not to be dump.