It's by design. You can target small surface objects when they fire at you, but it seems to only be for a very brief period. Skimmers also seem to take much more punishment than they could.
I'm thinking there's a bug here somewhere though. I've noticed you can't reliably target those massive gun turrets (not talking about the smaller defence turrets btw). The big ones show up red when they are actively firing and you target "some" of the time but it loses focus after a second or 2.
It's by design. You can target small surface objects when they fire at you, but it seems to only be for a very brief period. Skimmers also seem to take much more punishment than they could.