If they were for masterwork only, then they’d be on par with legendary shards. Everyone would have 100s of them and it deflates their economy. I can agree that 3 mw cores for infusion is a lot, but at the same time, if it cost no masterwork cores, then every weapon you owned would be masterworked out, and it takes away from the whole idea of masterworking your best and having it feel more special in your inventory