Template:Isda Default Rate summ
Default interest is one of those perennial things in finance and is generally a rate higher than the implied funding rate for the period and person in question. You might well ask — though one might, as the JC does, struggle heroically to not go there — whether an arbitrary loading on what ought to be a fair estimate of one’s actual carrying cost is not an unenforceable penalty, but hey, everyone does it.
Since for a default period you are not lending at term or on a fixed rate, but rather on an overnight floating rate, the sudden extension of that period for an indefinite but which you are “lending” money does not cause you any unexpected funding costs that would not be built into your ordinary floating rate.