The Rivash (siman 387) says that one must always endure a financial loss in order not to violate a lo sa’aseh – even if it is violated passively.
According to those who hold that one must generally not be forced to endure a financial loss in order to prevent passively transgressing a lo sa’aseh, we must understand why the lav of lo saguru (not fearing the litigants) is different.
Rabbeinu Yona, in Sha’arei Teshuvah 3:33, says that Hashem commanded that a judge is not to fear the litigants because, as the pasuk there says, the judgment is God’s. A dayan is doing the work of Hashem and therefore bears a greater responsibility. Hashem has therefore commanded dayanim to believe that no harm would come to them as a result of their work. Perhaps this is the reason why they must endure a financial loss in this situation – even though the action is a passive transgression.