Another interpretation: "before them" (Exodus 21:1) - and not before laymen. He said to him: We act as their agents [as agents of the ordained judges of the Land of Israel], just as is the case with admissions and loans. If so, [why not also] robberies and injuries? When we act as their agents, it is in a matter that is common; in a matter that is not common we do not act as their agents.
How Lay Judges Act as Agents of the Ancient Court
Yalkut Shimoni on Torah 308:3
דָּבָר אַחֵר, לִפְנֵיהֶם, וְלֹא לִפְנֵי הֶדְיוֹטוֹת. אֲמַר לֵיהּ, אֲנַן שְׁלִיחוּתַיְיהוּ קָא עַבְדִינַן, מִידִי דְּהָוֵה אֲהוֹדָאוֹת וְהַלְוָאוֹת. אִי הָכֵי, גְּזֵלוֹת וְחַבָּלוֹת נַמֵּי. כִּי עַבְדִינַן שְׁלִיחוּתַיְיהוּ בְּמִלְּתָא דִשְׁכִיחָא, בְּמִלְתָא דְּלָא שְׁכִיחָא לָא עָבְדִינַן שְׁלִיחוּתַיְיהוּ.