How do you expect such exchange to be maintained, pay taxes and survive? Through donations? I wouldn't advice anyone to depend on businesses that claim to offer free services but with no source of income.
There is a lot hype around exchange expenses. Servers are cheap. Huge exchange can easily operated by 20 people max!
And you forget about extra services.
For example pay for priority of order matching. Most of exchanges use price-time model. As extra service instead of "price-time" model use "price-transaction fee - time" matching model. So first will be matched the same price orders with the highest fee and than only orders with zero fee.
Taxes depends on country the exchange is registered at. For example Estonia or Cayman Islands. So free exchange with extra services could be very profitable.