I'd have to get that answer from dev - it's a good question. Will ask what the design intent was there.
Talked to the team about this.
As I suspected, similar to the philosophy I mentioned in my earlier post, this is due to the fact they can be used to make an output even if you don't get to keep it.
On top of that, it also meant Protean Planks could be earned to progress the Construction Contracts store which is in another sense an output. That's out of keeping with Protean Bars for example, which don't award Respect in the Artisan Workshop when used.