These items were originally released in-game during the XIII event back in ARR, when one-handed staves were still being made/used, primarily as a carryover from 1.0 (which had both one-handed and two-handed staves). So they were introduced during a time when it was still possible to glamour them onto one-handed weapons that could actually be used (for the most part - ARR primals have one-handed weapons one could use as a level-capped weapon).

That said, I understand the frustration. I believe it's been asked before to allow one-handed staves for both CNJ/WHM and THM/BLM to be glamoured over two-handed ones, but the developers have never implemented the feature. Can't recall if they ever gave a reason for it.