Maybe I've misunderstood you, but you wouldn't in any sense "pay" to JAK the opportunity cost of putting your money in their bank, so it can't really be called a fee. Opportunity cost is a thing unearned, not a thing paid.
If they were using the deposits to make interest earning investments for profit then that would be a slightly different matter, but they are not, they are using it to make interest free loans.
The key point is that they do not make profit. They could do this at any interest level, from zero up, as long as their deposits balanced their loans. If they chose to make a profit then they would need an imbalance between the interest on their loans and the interest on their deposits, and then there would be a net interest charged. The other sources of interest are inflation and defaults, and from what I can work out they do make investments with some of the deposits, precisely to cover these liabilities.
If they were using the deposits to make interest earning investments for profit then that would be a slightly different matter, but they are not, they are using it to make interest free loans.
The key point is that they do not make profit. They could do this at any interest level, from zero up, as long as their deposits balanced their loans. If they chose to make a profit then they would need an imbalance between the interest on their loans and the interest on their deposits, and then there would be a net interest charged. The other sources of interest are inflation and defaults, and from what I can work out they do make investments with some of the deposits, precisely to cover these liabilities.