As it has been said before, 0% fees ( first month ) is just a special limit offer for first clients. Fees are important to keep exchange running and growing,because there are employees, servers and all the other expenses required to keep the business live and nothing comes for free, specially when it comes to margin trading when exchange borrows you money to increase your order value, interest is required.
As customers should realize this fact and don't expect companies to give a service that costs them money for 100% free.
nothing comes for free, all those exchanges have employees that need to be paid, a permanent 0 zero fee is far from true, they take fees from you without realizing that, in rate exchange etc ...
In PrimeXBT case, they didn't claim to be a permanent 0 zero fee. they are just giving one month for first clients and after that If you bring 10 friends to PrimeXBT, you will get one more month of free trading!