If you don’t care about collateral damage and civilian casualties, why use the good (and expensive) stuff?
Besides not needing to use their PGMs, I’m sure protecting knowlege of their precision capabilities under real combat conditions is part of Russias decision process.
Because you'll miss your target? Allied bombers used to go out in 1000 aircraft formations because that's what they needed to do in order to hit their targets. A couple of dozen aircraft aren't sufficient to either terrorize enemy civilians into submission (something that 1000-aircraft* Allied firebombings were never able to do, anyway) or hit enemy military targets. If the Russians are using dumb bombs, they are delivering more airstrikes than Assad could on his own, but they are not giving him a capability he doesn't already have.
* Each B-17 might carry 8 500lb bombs, meaning that a 1000-bomber raid would result in 8,000 bombs being dropped in a matter of an hour or so, vs the dozens that can be dropped from the few Russian aircraft deployed.