Ultimately not my ruling, just my opinion. The IR is designed for players placed on the Injured list, which doesn’t exist in spring training unless they’re 60 day DL players and won’t be back and healthy in the next day or two. Allowing this would require constant tracking of rosters to pull players off the IL when they’re not hurt anymore.
This just allows anyone to roster more players than the roster allows, allowing decisions to be made closer to the season when more information is available.