With Ripple, can I send IOU without setting a trust? I have a simple question. User A wants to send to user B some IOU. There is no trust between A and B. There is no possible risk for user B. Can this be done without establishing a trust line? If no, why this is not allowed?
To pay someone, say, $10, you must deliver to them an asset that they have decided is worth $10. A person can never be made to hold an asset they did not agree to hold. These are core Ripple design rules that apply to all IOUs. The main reason for not allowing the "harmless" payment is that it might not be clear to the recipient that the payment is one he is free to ignore and this can create complexity for the recipient who may be legally obligated, for example, to refund it.

This was a close call though. We could have allowed this kind of payment with a special flag and designed the client to show those payments in a special way, and we could have added an account flag to indicate that you refuse these kinds of payments The added complexity and risk of confusion didn't seem to be justified by any significant benefit.