Could be a scheduling thing (they don't want to disrupt the GSA/GSTL rotation by giving one of them LOD shifts), could be a store management thing (the STL has tried Sr.-GSTLs before and it didn't work out for some reason), could even be a district policy (same as the last explanation but insert DTL in place of STL). Who knows?
It's a stupid policy, I think, but that doesn't necessarily mean they were actually lying about it. It's not a company policy, but that's all we can really say for sure.
I can understand why though. If you were scheduled as GSTL, you'd be stuck at the front end, but the LOD has to be available for the whole store. Either you'd have to be scheduled in a different workcenter (i.e. Hardlines) for your LOD shifts or you'd need double coverage at the front end.
A couple years ago, when one of our SrTLs was on LOA, one of our GSTLs was temporarily a SrTL. On his LOD days, he was scheduled in Hardlines instead of GSTL.