I think its due to the fact that construction frigates are produced by the planet for free. So why bother repairing one? they are reproduced very quickly.
Well, while the planet is in combat you have 6 minutes between construction frigate spawns, which is a long time. A lot of times it is advantageous to repair one. If a player targets it you might as well forget about trying, but the AI usually doesn't go for them and slowly nibbles away from secondary capital ship fire, AEs, etc. In those cases repairing them is a lot more beneficial than watching it melt away slowly and not come back for 6 minutes
That said, I don't think it's either a bug or omission, but since the construction ship is not classified as a Frigate, Capital, or structure it's omitted as a valid target for the ability in the engine.
