Well, it can be called a kind of paradox. Of course, there are projects that manage to conduct IEO for free, they pay part of their coins to the exchange. But as a rule, there is no trading volume on such exchanges.
If the project manage to conduct IEO for free and pay tokens to exchange usualy it is not a good exchange and it is very difficult to collect money during IEO and usualy after IEO when the listing starts the price of the tokens falls and tokens becomes shitcoins.