The trading commission is 0,8% (0,4% two ways), and it is lower the more you have volume. In addition, they have withdrawal fees.
In addition to operational costs probably fraud is also a big cost, but it is very difficult to estimate how big it is for them.
And, they don't exactly need to take it from their profits, they can just do fractional reserve... They are a bank, as are any bitcoin services that hold money for the users.
They are not regulated as a bank, they cant "just do fractional reserve" which you entirely misunderstand. They would be stealing from their customers.
In addition to operational costs probably fraud is also a big cost, but it is very difficult to estimate how big it is for them.
And, they don't exactly need to take it from their profits, they can just do fractional reserve... They are a bank, as are any bitcoin services that hold money for the users.