I can think of one possible reason the Russians are letting the “irregulars” run amok.

The remaining elements of the Georgian Army seem to be holding strong defensive positions on the road to Tbilisi. It must be difficult for them to sit there while the rest of the country is raped. If they can be drawn into the open to attack the brigands, that achieves two things:

(1) It weakens their defensive position and exposes them
(2) It provides a pretext for Putin to say that the Georgians have broken the ceasefire

So it could be part of a strategy to take Tbilisi, further weaken the Georgian military, or to create a propaganda incident.