I think its more, they only riposte ONCE.
Hence, for a stoner to deal the killing blow, probably they already got hit by a stone or dagger previously.
Meanwhile kobolds deal 3 damage, and so the killing blow is also the first one.
Ah yes that was it, thanks! Thus the only way to prevent a Riposte is to kill them in melee before they get the chance, or BATS! them.
