What was the case against not letting them alter it at all once set? As in, if they want to start issuing new debts with a new fee they can create a new address. My issue is that the cost of transferring the debt would be a key ingredient in how I value it. One of the better things I'm seeing in Ripple is that it would give debt issuers less control over the transference of their debt, and less ability to suddenly make arbitrary rules or fees about how that happens. Fees would be set up front. They can charge a fee, perhaps they can even decrease it, but I don't see why you would allow them increase it. Particularly as you (being Ripple) ultimately want people to keep their IOUs in the system rather than cash them out.
Like I said, fee or no fee the gateway can always change the terms of the settlement agreement (although they might be breaking the law). So preventing the transaction fee from increasing at the protocol level is not particularly constraining.
To be clear, what I'm saying is that even if Ripple prevents transfer fees from rising, you could still get an email from your gateway that says "Our new policy on redemption of IOUs is to pay 95% of the face value of the IOUs." In the worst case scenario (i.e. default) this redemption amount would be 0%. The point is that putting protocol restrictions on transfer fees doesn't completely solve the "problem."